diff --git a/src/auth/commands/create-command.ts b/src/auth/commands/create-command.ts
index 3dd55c84..03668b47 100644
--- a/src/auth/commands/create-command.ts
+++ b/src/auth/commands/create-command.ts
@@ -42,7 +42,7 @@ export async function handleCreate(ctx: CommandContext, args: string[]): Promise
   try {
     // Create instance directory
     console.log(info(`Creating profile: ${profileName}`));
-    const instancePath = ctx.instanceMgr.ensureInstance(profileName);
+    const instancePath = await ctx.instanceMgr.ensureInstance(profileName);
 
     // Create/update profile entry based on config mode
     if (isUnifiedMode()) {
diff --git a/src/ccs.ts b/src/ccs.ts
index 5676fd96..dc673635 100644
--- a/src/ccs.ts
+++ b/src/ccs.ts
@@ -884,7 +884,7 @@ async function main(): Promise<void> {
       const instanceMgr = new InstanceManager();
 
       // Ensure instance exists (lazy init if needed)
-      const instancePath = instanceMgr.ensureInstance(profileInfo.name);
+      const instancePath = await instanceMgr.ensureInstance(profileInfo.name);
 
       // Update last_used timestamp (check unified config first, fallback to legacy)
       if (registry.hasAccountUnified(profileInfo.name)) {
diff --git a/src/management/instance-manager.ts b/src/management/instance-manager.ts
index 2063a39c..a9b273bd 100644
--- a/src/management/instance-manager.ts
+++ b/src/management/instance-manager.ts
@@ -26,7 +26,7 @@ class InstanceManager {
   /**
    * Ensure instance exists for profile (lazy init only)
    */
-  ensureInstance(profileName: string): string {
+  async ensureInstance(profileName: string): Promise<string> {
     const instancePath = this.getInstancePath(profileName);
 
     // Lazy initialization
@@ -37,6 +37,9 @@ class InstanceManager {
     // Validate structure (auto-fix missing dirs)
     this.validateInstance(instancePath);
 
+    // Keep project memory shared across instances.
+    await this.sharedManager.syncProjectMemories(instancePath);
+
     return instancePath;
   }
 
diff --git a/src/management/shared-manager.ts b/src/management/shared-manager.ts
index 754e42e7..7871e95d 100644
--- a/src/management/shared-manager.ts
+++ b/src/management/shared-manager.ts
@@ -201,6 +201,98 @@ class SharedManager {
     this.normalizePluginRegistryPaths();
   }
 
+  /**
+   * Ensure all project memory directories for an instance are shared.
+   *
+   * Source layout (isolated):
+   *   ~/.ccs/instances/<profile>/projects/<project>/memory/
+   *
+   * Shared layout (canonical):
+   *   ~/.ccs/shared/memory/<project>/
+   */
+  async syncProjectMemories(instancePath: string): Promise<void> {
+    const projectsDir = path.join(instancePath, 'projects');
+    if (!(await this.pathExists(projectsDir))) {
+      return;
+    }
+
+    await this.ensureDirectory(this.sharedDir);
+
+    const sharedMemoryRoot = path.join(this.sharedDir, 'memory');
+    await this.ensureDirectory(sharedMemoryRoot);
+
+    let projectEntries: fs.Dirent[] = [];
+    try {
+      projectEntries = await fs.promises.readdir(projectsDir, { withFileTypes: true });
+    } catch (_err) {
+      return;
+    }
+
+    const projects = projectEntries.filter((entry) => entry.isDirectory());
+    if (projects.length === 0) {
+      return;
+    }
+
+    let migrated = 0;
+    let merged = 0;
+    let linked = 0;
+    const instanceName = path.basename(instancePath);
+
+    for (const project of projects) {
+      const projectDir = path.join(projectsDir, project.name);
+      const projectMemoryPath = path.join(projectDir, 'memory');
+      const sharedProjectMemoryPath = path.join(sharedMemoryRoot, project.name);
+
+      const projectMemoryStats = await this.getLstat(projectMemoryPath);
+      if (!projectMemoryStats) {
+        if (await this.ensureProjectMemoryLink(projectMemoryPath, sharedProjectMemoryPath)) {
+          linked++;
+        }
+        continue;
+      }
+
+      if (projectMemoryStats.isSymbolicLink()) {
+        if (await this.isSymlinkTarget(projectMemoryPath, sharedProjectMemoryPath)) {
+          continue;
+        }
+
+        await fs.promises.unlink(projectMemoryPath);
+        if (await this.ensureProjectMemoryLink(projectMemoryPath, sharedProjectMemoryPath)) {
+          linked++;
+        }
+        continue;
+      }
+
+      if (!projectMemoryStats.isDirectory()) {
+        continue;
+      }
+
+      if (!(await this.pathExists(sharedProjectMemoryPath))) {
+        await this.moveDirectory(projectMemoryPath, sharedProjectMemoryPath);
+        migrated++;
+      } else {
+        merged += await this.mergeDirectoryWithConflictCopies(
+          projectMemoryPath,
+          sharedProjectMemoryPath,
+          instanceName
+        );
+        await fs.promises.rm(projectMemoryPath, { recursive: true, force: true });
+      }
+
+      if (await this.ensureProjectMemoryLink(projectMemoryPath, sharedProjectMemoryPath)) {
+        linked++;
+      }
+    }
+
+    if (migrated > 0 || merged > 0 || linked > 0) {
+      console.log(
+        ok(
+          `Synced shared project memory: ${migrated} migrated, ${merged} merged conflict(s), ${linked} linked`
+        )
+      );
+    }
+  }
+
   /**
    * Normalize plugin registry paths to use canonical ~/.claude/ paths
    * instead of instance-specific ~/.ccs/instances/<name>/ paths.
@@ -420,6 +512,190 @@ class SharedManager {
     console.log(ok(`Migrated ${migrated} instance(s), skipped ${skipped}`));
   }
 
+  /**
+   * Ensure memory path is linked to shared memory root.
+   * Returns true when a link/copy was created or updated.
+   */
+  private async ensureProjectMemoryLink(linkPath: string, targetPath: string): Promise<boolean> {
+    await this.ensureDirectory(targetPath);
+
+    const linkStats = await this.getLstat(linkPath);
+    if (linkStats) {
+      if (linkStats.isSymbolicLink() && (await this.isSymlinkTarget(linkPath, targetPath))) {
+        return false;
+      }
+
+      if (linkStats.isDirectory()) {
+        await fs.promises.rm(linkPath, { recursive: true, force: true });
+      } else {
+        await fs.promises.unlink(linkPath);
+      }
+    }
+
+    const symlinkType: 'dir' | 'junction' = process.platform === 'win32' ? 'junction' : 'dir';
+    const linkTarget = process.platform === 'win32' ? path.resolve(targetPath) : targetPath;
+
+    try {
+      await fs.promises.symlink(linkTarget, linkPath, symlinkType);
+      return true;
+    } catch (_err) {
+      if (process.platform === 'win32') {
+        this.copyDirectoryFallback(targetPath, linkPath);
+        console.log(
+          warn(`Symlink failed for project memory, copied instead (enable Developer Mode)`)
+        );
+        return true;
+      }
+      throw _err;
+    }
+  }
+
+  /**
+   * Check whether symlink points to expected target.
+   */
+  private async isSymlinkTarget(linkPath: string, expectedTarget: string): Promise<boolean> {
+    try {
+      const stats = await fs.promises.lstat(linkPath);
+      if (!stats.isSymbolicLink()) {
+        return false;
+      }
+
+      const currentTarget = await fs.promises.readlink(linkPath);
+      const resolvedCurrentTarget = path.resolve(path.dirname(linkPath), currentTarget);
+      const resolvedExpectedTarget = path.resolve(expectedTarget);
+      return resolvedCurrentTarget === resolvedExpectedTarget;
+    } catch (_err) {
+      return false;
+    }
+  }
+
+  /**
+   * Move directory, with cross-device fallback.
+   */
+  private async moveDirectory(src: string, dest: string): Promise<void> {
+    try {
+      await fs.promises.rename(src, dest);
+    } catch (err) {
+      const error = err as NodeJS.ErrnoException;
+      if (error.code !== 'EXDEV') {
+        throw err;
+      }
+
+      await fs.promises.cp(src, dest, { recursive: true });
+      await fs.promises.rm(src, { recursive: true, force: true });
+    }
+  }
+
+  /**
+   * Merge source into target. On file conflicts, keep target and copy source
+   * as "<name>.migrated-from-<instance>[-N]" to avoid data loss.
+   */
+  private async mergeDirectoryWithConflictCopies(
+    sourceDir: string,
+    targetDir: string,
+    instanceName: string
+  ): Promise<number> {
+    await this.ensureDirectory(targetDir);
+
+    let conflicts = 0;
+    const entries = await fs.promises.readdir(sourceDir, { withFileTypes: true });
+
+    for (const entry of entries) {
+      const sourcePath = path.join(sourceDir, entry.name);
+      const targetPath = path.join(targetDir, entry.name);
+
+      if (entry.isDirectory()) {
+        conflicts += await this.mergeDirectoryWithConflictCopies(
+          sourcePath,
+          targetPath,
+          instanceName
+        );
+        continue;
+      }
+
+      if (entry.isFile()) {
+        if (!(await this.pathExists(targetPath))) {
+          await fs.promises.copyFile(sourcePath, targetPath);
+          continue;
+        }
+
+        if (await this.fileContentsEqual(sourcePath, targetPath)) {
+          continue;
+        }
+
+        const conflictPath = await this.getConflictCopyPath(targetPath, instanceName);
+        await fs.promises.copyFile(sourcePath, conflictPath);
+        conflicts++;
+      }
+    }
+
+    return conflicts;
+  }
+
+  /**
+   * Compare two files byte-for-byte.
+   */
+  private async fileContentsEqual(fileA: string, fileB: string): Promise<boolean> {
+    try {
+      const [statA, statB] = await Promise.all([fs.promises.stat(fileA), fs.promises.stat(fileB)]);
+      if (statA.size !== statB.size) {
+        return false;
+      }
+
+      const [contentA, contentB] = await Promise.all([
+        fs.promises.readFile(fileA),
+        fs.promises.readFile(fileB),
+      ]);
+      return contentA.equals(contentB);
+    } catch (_err) {
+      return false;
+    }
+  }
+
+  /**
+   * Build a non-destructive conflict copy path.
+   */
+  private async getConflictCopyPath(
+    existingTargetPath: string,
+    instanceName: string
+  ): Promise<string> {
+    const safeInstanceName = instanceName.replace(/[^a-zA-Z0-9_-]/g, '-').toLowerCase();
+    const baseSuffix = `.migrated-from-${safeInstanceName}`;
+
+    let candidate = `${existingTargetPath}${baseSuffix}`;
+    let sequence = 1;
+    while (await this.pathExists(candidate)) {
+      candidate = `${existingTargetPath}${baseSuffix}-${sequence}`;
+      sequence++;
+    }
+
+    return candidate;
+  }
+
+  private async pathExists(targetPath: string): Promise<boolean> {
+    try {
+      await fs.promises.access(targetPath);
+      return true;
+    } catch (_err) {
+      return false;
+    }
+  }
+
+  private async ensureDirectory(targetPath: string): Promise<void> {
+    await fs.promises.mkdir(targetPath, { recursive: true, mode: 0o700 });
+  }
+
+  private async getLstat(targetPath: string): Promise<fs.Stats | null> {
+    try {
+      return await fs.promises.lstat(targetPath);
+    } catch (err) {
+      if ((err as NodeJS.ErrnoException).code === 'ENOENT') {
+        return null;
+      }
+      throw err;
+    }
+  }
+
   /**
    * Copy directory as fallback (Windows without Developer Mode)
    */
diff --git a/tests/unit/shared-memory-sync.test.ts b/tests/unit/shared-memory-sync.test.ts
new file mode 100644
index 00000000..a041209b
--- /dev/null
+++ b/tests/unit/shared-memory-sync.test.ts
@@ -0,0 +1,113 @@
+import { afterEach, beforeEach, describe, expect, it } from 'bun:test';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import SharedManager from '../../src/management/shared-manager';
+
+function getTestCcsDir(): string {
+  if (!process.env.CCS_HOME) {
+    throw new Error('CCS_HOME must be set in tests');
+  }
+  return path.join(path.resolve(process.env.CCS_HOME), '.ccs');
+}
+
+describe('SharedManager project memory sync', () => {
+  let tempRoot = '';
+  let originalHome: string | undefined;
+  let originalCcsHome: string | undefined;
+  let originalCcsDir: string | undefined;
+
+  beforeEach(() => {
+    tempRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'ccs-shared-memory-test-'));
+    originalHome = process.env.HOME;
+    originalCcsHome = process.env.CCS_HOME;
+    originalCcsDir = process.env.CCS_DIR;
+
+    const isolatedHome = path.join(tempRoot, 'home');
+    fs.mkdirSync(isolatedHome, { recursive: true });
+    process.env.HOME = isolatedHome;
+    process.env.CCS_HOME = tempRoot;
+    delete process.env.CCS_DIR;
+  });
+
+  afterEach(() => {
+    if (originalHome !== undefined) process.env.HOME = originalHome;
+    else delete process.env.HOME;
+
+    if (originalCcsHome !== undefined) process.env.CCS_HOME = originalCcsHome;
+    else delete process.env.CCS_HOME;
+
+    if (originalCcsDir !== undefined) process.env.CCS_DIR = originalCcsDir;
+    else delete process.env.CCS_DIR;
+
+    if (tempRoot && fs.existsSync(tempRoot)) {
+      fs.rmSync(tempRoot, { recursive: true, force: true });
+    }
+  });
+
+  it('migrates existing project memory and replaces it with a shared symlink', async () => {
+    const ccsDir = getTestCcsDir();
+    const instancePath = path.join(ccsDir, 'instances', 'work');
+    const projectName = '-tmp-my-project';
+    const projectMemoryPath = path.join(instancePath, 'projects', projectName, 'memory');
+    fs.mkdirSync(projectMemoryPath, { recursive: true });
+    fs.writeFileSync(path.join(projectMemoryPath, 'MEMORY.md'), 'instance knowledge', 'utf8');
+
+    const manager = new SharedManager();
+    await manager.syncProjectMemories(instancePath);
+
+    const sharedMemoryFile = path.join(ccsDir, 'shared', 'memory', projectName, 'MEMORY.md');
+    expect(fs.existsSync(sharedMemoryFile)).toBe(true);
+    expect(fs.readFileSync(sharedMemoryFile, 'utf8')).toBe('instance knowledge');
+
+    const linkStats = fs.lstatSync(projectMemoryPath);
+    expect(linkStats.isSymbolicLink()).toBe(true);
+
+    const resolvedTarget = path.resolve(path.dirname(projectMemoryPath), fs.readlinkSync(projectMemoryPath));
+    expect(resolvedTarget).toBe(path.join(ccsDir, 'shared', 'memory', projectName));
+  });
+
+  it('preserves canonical memory and writes conflict copy when contents differ', async () => {
+    const ccsDir = getTestCcsDir();
+    const instancePath = path.join(ccsDir, 'instances', 'work');
+    const projectName = '-tmp-shared-project';
+    const projectMemoryPath = path.join(instancePath, 'projects', projectName, 'memory');
+    const sharedProjectMemoryPath = path.join(ccsDir, 'shared', 'memory', projectName);
+    fs.mkdirSync(projectMemoryPath, { recursive: true });
+    fs.mkdirSync(sharedProjectMemoryPath, { recursive: true });
+
+    fs.writeFileSync(path.join(projectMemoryPath, 'MEMORY.md'), 'instance memory', 'utf8');
+    fs.writeFileSync(path.join(sharedProjectMemoryPath, 'MEMORY.md'), 'shared memory', 'utf8');
+
+    const manager = new SharedManager();
+    await manager.syncProjectMemories(instancePath);
+
+    const canonicalFile = path.join(sharedProjectMemoryPath, 'MEMORY.md');
+    expect(fs.readFileSync(canonicalFile, 'utf8')).toBe('shared memory');
+
+    const conflictFile = path.join(sharedProjectMemoryPath, 'MEMORY.md.migrated-from-work');
+    expect(fs.existsSync(conflictFile)).toBe(true);
+    expect(fs.readFileSync(conflictFile, 'utf8')).toBe('instance memory');
+
+    const linkStats = fs.lstatSync(projectMemoryPath);
+    expect(linkStats.isSymbolicLink()).toBe(true);
+  });
+
+  it('creates shared memory link for projects that do not have memory directory yet', async () => {
+    const ccsDir = getTestCcsDir();
+    const instancePath = path.join(ccsDir, 'instances', 'work');
+    const projectName = '-tmp-new-project';
+    const projectPath = path.join(instancePath, 'projects', projectName);
+    const projectMemoryPath = path.join(projectPath, 'memory');
+    fs.mkdirSync(projectPath, { recursive: true });
+
+    const manager = new SharedManager();
+    await manager.syncProjectMemories(instancePath);
+
+    const linkStats = fs.lstatSync(projectMemoryPath);
+    expect(linkStats.isSymbolicLink()).toBe(true);
+
+    const sharedProjectMemoryPath = path.join(ccsDir, 'shared', 'memory', projectName);
+    expect(fs.existsSync(sharedProjectMemoryPath)).toBe(true);
+  });
+});