diff --git a/packages/vitest/src/node/specifications.ts b/packages/vitest/src/node/specifications.ts
index 54fe67e7544b..43317ae44e8e 100644
--- a/packages/vitest/src/node/specifications.ts
+++ b/packages/vitest/src/node/specifications.ts
@@ -150,9 +150,10 @@ export class VitestSpecifications {
       return []
     }
 
+    const transformCache = new Map<string, string[]>()
     const testGraphs = await Promise.all(
       specs.map(async (spec) => {
-        const deps = await this.getTestDependencies(spec)
+        const deps = await this.getTestDependencies(spec, transformCache)
         return [spec, deps] as const
       }),
     )
@@ -169,19 +170,31 @@ export class VitestSpecifications {
     return runningTests
   }
 
-  private async getTestDependencies(spec: TestSpecification, deps = new Set<string>()): Promise<Set<string>> {
+  private async getTestDependencies(
+    spec: TestSpecification,
+    transformCache?: Map<string, string[]>,
+    deps = new Set<string>(),
+  ): Promise<Set<string>> {
     const addImports = async (project: TestProject, filepath: string) => {
       if (deps.has(filepath)) {
         return
       }
       deps.add(filepath)
 
-      const mod = project.vite.environments.ssr.moduleGraph.getModuleById(filepath)
-      const transformed = mod?.transformResult || await project.vite.environments.ssr.transformRequest(filepath)
-      if (!transformed) {
-        return
+      let dependencies: string[]
+      const cached = transformCache?.get(filepath)
+      if (cached) {
+        dependencies = cached
+      }
+      else {
+        const mod = project.vite.environments.ssr.moduleGraph.getModuleById(filepath)
+        const transformed = mod?.transformResult || await project.vite.environments.ssr.transformRequest(filepath)
+        if (!transformed) {
+          return
+        }
+        dependencies = [...transformed.deps || [], ...transformed.dynamicDeps || []]
+        transformCache?.set(filepath, dependencies)
       }
-      const dependencies = [...transformed.deps || [], ...transformed.dynamicDeps || []]
       await Promise.all(dependencies.map(async (dep) => {
         const fsPath = dep.startsWith('/@fs/')
           ? dep.slice(isWindows ? 5 : 4)
diff --git a/test/cli/fixtures/related-multi-project/src/shared.ts b/test/cli/fixtures/related-multi-project/src/shared.ts
new file mode 100644
index 000000000000..41d988ae6d5f
--- /dev/null
+++ b/test/cli/fixtures/related-multi-project/src/shared.ts
@@ -0,0 +1 @@
+export const value = 'shared'
diff --git a/test/cli/fixtures/related-multi-project/src/unrelated.ts b/test/cli/fixtures/related-multi-project/src/unrelated.ts
new file mode 100644
index 000000000000..a236f932c1e8
--- /dev/null
+++ b/test/cli/fixtures/related-multi-project/src/unrelated.ts
@@ -0,0 +1 @@
+export const other = 'unrelated'
diff --git a/test/cli/fixtures/related-multi-project/tests/jsdom.test.ts b/test/cli/fixtures/related-multi-project/tests/jsdom.test.ts
new file mode 100644
index 000000000000..4cfb0050ae82
--- /dev/null
+++ b/test/cli/fixtures/related-multi-project/tests/jsdom.test.ts
@@ -0,0 +1,6 @@
+import { expect, test } from 'vitest'
+import { value } from '../src/shared'
+
+test('jsdom project uses shared module', () => {
+  expect(value).toBe('shared')
+})
diff --git a/test/cli/fixtures/related-multi-project/tests/node.test.ts b/test/cli/fixtures/related-multi-project/tests/node.test.ts
new file mode 100644
index 000000000000..df1614056789
--- /dev/null
+++ b/test/cli/fixtures/related-multi-project/tests/node.test.ts
@@ -0,0 +1,6 @@
+import { expect, test } from 'vitest'
+import { value } from '../src/shared'
+
+test('node project uses shared module', () => {
+  expect(value).toBe('shared')
+})
diff --git a/test/cli/fixtures/related-multi-project/tests/unrelated.test.ts b/test/cli/fixtures/related-multi-project/tests/unrelated.test.ts
new file mode 100644
index 000000000000..1320418532e7
--- /dev/null
+++ b/test/cli/fixtures/related-multi-project/tests/unrelated.test.ts
@@ -0,0 +1,6 @@
+import { expect, test } from 'vitest'
+import { other } from '../src/unrelated'
+
+test('unrelated test should not run', () => {
+  expect(other).toBe('unrelated')
+})
diff --git a/test/cli/fixtures/related-multi-project/vitest.config.ts b/test/cli/fixtures/related-multi-project/vitest.config.ts
new file mode 100644
index 000000000000..b3e6c5cc1fbb
--- /dev/null
+++ b/test/cli/fixtures/related-multi-project/vitest.config.ts
@@ -0,0 +1,22 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    projects: [
+      {
+        test: {
+          name: 'project-jsdom',
+          environment: 'jsdom',
+          include: ['./tests/jsdom.test.ts'],
+        },
+      },
+      {
+        test: {
+          name: 'project-node',
+          environment: 'node',
+          include: ['./tests/node.test.ts', './tests/unrelated.test.ts'],
+        },
+      },
+    ],
+  },
+})
diff --git a/test/cli/test/related-multi-project.test.ts b/test/cli/test/related-multi-project.test.ts
new file mode 100644
index 000000000000..22ac162349e4
--- /dev/null
+++ b/test/cli/test/related-multi-project.test.ts
@@ -0,0 +1,38 @@
+import { resolve } from 'pathe'
+import { describe, expect, it } from 'vitest'
+import { runVitest } from '../../test-utils'
+
+// Regression test for https://github.com/vitest-dev/vitest/issues/9855
+// Verifies that `vitest related` correctly resolves dependencies across
+// multiple projects. The cache poisoning aspect of the bug (shared plugin
+// caches corrupted by redundant transforms) requires @vitejs/plugin-vue
+// with browser mode to reproduce and was verified against a real project.
+describe('related with multiple projects', () => {
+  it('correctly finds related tests across multiple projects', async () => {
+    const root = resolve(import.meta.dirname, '../fixtures/related-multi-project')
+
+    const { stdout, stderr } = await runVitest({
+      root,
+      related: resolve(root, 'src/shared.ts'),
+      passWithNoTests: true,
+    })
+
+    expect(stderr).toBe('')
+    expect(stdout).toContain('project-jsdom')
+    expect(stdout).toContain('project-node')
+    expect(stdout).toContain('2 passed')
+  })
+
+  it('does not run unrelated tests', async () => {
+    const root = resolve(import.meta.dirname, '../fixtures/related-multi-project')
+
+    const { stdout, stderr } = await runVitest({
+      root,
+      related: resolve(root, 'src/shared.ts'),
+      passWithNoTests: true,
+    })
+
+    expect(stderr).toBe('')
+    expect(stdout).not.toContain('unrelated')
+  })
+})