diff --git a/.changeset/eff-428-pg-transaction-permit.md b/.changeset/eff-428-pg-transaction-permit.md
new file mode 100644
index 00000000000..be6a9a20e9f
--- /dev/null
+++ b/.changeset/eff-428-pg-transaction-permit.md
@@ -0,0 +1,5 @@
+---
+"@effect/sql-pg": patch
+---
+
+Hold the shared PostgreSQL client permit for the full transaction lifetime.
diff --git a/packages/sql/pg/src/PgClient.ts b/packages/sql/pg/src/PgClient.ts
index 4e33ab77a8d..9003a925aec 100644
--- a/packages/sql/pg/src/PgClient.ts
+++ b/packages/sql/pg/src/PgClient.ts
@@ -522,6 +522,17 @@ export const fromClient = Effect.fnUntraced(function*(
     )
   const connection = makeConection(client)
   const acquirer = semaphore.withPermit(Effect.succeed(connection))
+  const transactionAcquirer = Effect.uninterruptibleMask((restore) => {
+    const fiber = Fiber.getCurrent()!
+    const scope = Context.getUnsafe(fiber.context, Scope.Scope)
+    return Effect.as(
+      Effect.tap(
+        restore(semaphore.take(1)),
+        () => Scope.addFinalizer(scope, semaphore.release(1))
+      ),
+      connection
+    )
+  })
 
   const config: PgClientConfig = {
     ...options,
@@ -535,7 +546,7 @@ export const fromClient = Effect.fnUntraced(function*(
 
   return yield* makeWith({
     acquirer,
-    transactionAcquirer: acquirer,
+    transactionAcquirer,
     listenAcquirer: streamClient,
     config,
     spanAttributes: options.spanAttributes,
diff --git a/packages/sql/pg/test/Client.integration.test.ts b/packages/sql/pg/test/Client.integration.test.ts
index 78b4ed2c111..4db2866fffd 100644
--- a/packages/sql/pg/test/Client.integration.test.ts
+++ b/packages/sql/pg/test/Client.integration.test.ts
@@ -1,7 +1,8 @@
 import { PgClient } from "@effect/sql-pg"
 import { assert, expect, it } from "@effect/vitest"
-import { Deferred, Effect, Fiber, Redacted, Stream, String } from "effect"
+import { Deferred, Effect, Fiber, Option, Redacted, Stream, String } from "effect"
 import { TestClock } from "effect/testing"
+import * as Reactivity from "effect/unstable/reactivity/Reactivity"
 import { SqlClient } from "effect/unstable/sql"
 import * as Statement from "effect/unstable/sql/Statement"
 import { parse as parsePgConnectionString } from "pg-connection-string"
@@ -413,3 +414,51 @@ it.layer(PgContainer.layerClientSingleConnection, { timeout: "30 seconds" })("Pg
       expect(Array.from(payloads)).toEqual(["payload"])
     }).pipe(TestClock.withLive), 20_000)
 })
+
+it.effect("serializes transactions that share one pg.Client", () =>
+  Effect.gen(function*() {
+    const secondBegin = yield* Deferred.make<void>()
+    const firstBodyStarted = yield* Deferred.make<void>()
+    const releaseFirstBody = yield* Deferred.make<void>()
+    let beginCalls = 0
+    const pg = {
+      host: "localhost",
+      port: 5432,
+      database: "postgres",
+      user: "postgres",
+      password: undefined,
+      ssl: false,
+      on() {},
+      off() {},
+      query(sql: string, _params: ReadonlyArray<unknown>, callback: (error: null, result: unknown) => void) {
+        if (sql === "BEGIN" && ++beginCalls === 2) {
+          Deferred.doneUnsafe(secondBegin, Effect.void)
+        }
+        callback(null, { rows: [] })
+      }
+    }
+
+    const sql = yield* PgClient.fromClient({
+      acquire: Effect.succeed(pg as any),
+      acquireForStream: false
+    })
+    const first = yield* sql.withTransaction(Effect.gen(function*() {
+      yield* Deferred.succeed(firstBodyStarted, undefined)
+      yield* Deferred.await(releaseFirstBody)
+    })).pipe(Effect.forkScoped)
+    yield* Deferred.await(firstBodyStarted)
+    const second = yield* sql.withTransaction(Effect.void).pipe(Effect.forkScoped)
+
+    const overlap = yield* Deferred.await(secondBegin).pipe(
+      Effect.timeoutOption("100 millis"),
+      TestClock.withLive
+    )
+    yield* Deferred.succeed(releaseFirstBody, undefined)
+    yield* Fiber.join(first)
+    yield* Fiber.join(second)
+
+    assert.isTrue(Option.isNone(overlap))
+  }).pipe(
+    Effect.scoped,
+    Effect.provide(Reactivity.layer)
+  ))