diff --git a/.changeset/slimy-mugs-taste.md b/.changeset/slimy-mugs-taste.md
new file mode 100644
index 0000000000..5773db6853
--- /dev/null
+++ b/.changeset/slimy-mugs-taste.md
@@ -0,0 +1,5 @@
+---
+'@backstage/backend-test-utils': patch
+---
+
+Fix PostgreSQL 18 `TestDatabases` by pinning the data directory
diff --git a/packages/backend-defaults/src/entrypoints/scheduler/lib/TaskWorker.test.ts b/packages/backend-defaults/src/entrypoints/scheduler/lib/TaskWorker.test.ts
index de1b6e4caf..5bd3fb38cb 100644
--- a/packages/backend-defaults/src/entrypoints/scheduler/lib/TaskWorker.test.ts
+++ b/packages/backend-defaults/src/entrypoints/scheduler/lib/TaskWorker.test.ts
@@ -46,7 +46,7 @@ describe('TaskWorker', () => {
       const settings: TaskSettingsV2 = {
         version: 2,
         cadence: '*/2 * * * * *',
-        initialDelayDuration: Duration.fromObject({ seconds: 1 }).toISO()!,
+        initialDelayDuration: Duration.fromObject({ seconds: 3 }).toISO()!, // over 1 second, to cover for 1-second granularity in some database timestamps
         timeoutAfterDuration: Duration.fromObject({ minutes: 1 }).toISO()!,
       };
 
@@ -65,7 +65,7 @@ describe('TaskWorker', () => {
       expect(JSON.parse(row.settings_json)).toEqual({
         version: 2,
         cadence: '*/2 * * * * *',
-        initialDelayDuration: 'PT1S',
+        initialDelayDuration: 'PT3S',
         timeoutAfterDuration: 'PT1M',
       });
       await expect(TaskWorker.taskStates(knex)).resolves.toEqual(
@@ -80,6 +80,10 @@ describe('TaskWorker', () => {
         ]),
       );
 
+      // Note that this is timing sensitive - if things take too long between
+      // persistTask and findReadyTask, this test will fail. We set some margin
+      // of initialDelayDuration to cover for this, but if this turns out to be
+      // flaky, we may have to revisit how this test is written.
       await expect(worker.findReadyTask()).resolves.toEqual({
         result: 'not-ready-yet',
       });
diff --git a/packages/backend-test-utils/package.json b/packages/backend-test-utils/package.json
index 70e74df86b..cd4f68ead0 100644
--- a/packages/backend-test-utils/package.json
+++ b/packages/backend-test-utils/package.json
@@ -75,7 +75,7 @@
     "mysql2": "^3.0.0",
     "pg": "^8.11.3",
     "pg-connection-string": "^2.3.0",
-    "testcontainers": "^10.0.0",
+    "testcontainers": "^11.9.0",
     "text-extensions": "^2.4.0",
     "uuid": "^11.0.0",
     "yn": "^4.0.0",
diff --git a/packages/backend-test-utils/src/database/postgres.ts b/packages/backend-test-utils/src/database/postgres.ts
index e6122b9ab7..d7238cb36a 100644
--- a/packages/backend-test-utils/src/database/postgres.ts
+++ b/packages/backend-test-utils/src/database/postgres.ts
@@ -77,7 +77,11 @@ export async function startPostgresContainer(image: string): Promise<{
 
   const container = await new GenericContainer(image)
     .withExposedPorts(5432)
-    .withEnvironment({ POSTGRES_PASSWORD: password })
+    .withEnvironment({
+      // Since postgres 18, the default directory changed - so we pin it here
+      PGDATA: '/var/lib/postgresql/data',
+      POSTGRES_PASSWORD: password,
+    })
     .withTmpFs({ '/var/lib/postgresql/data': 'rw' })
     .start();
 
diff --git a/yarn.lock b/yarn.lock
index 0f910f4340..1b379e92da 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3097,7 +3097,7 @@ __metadata:
     pg: "npm:^8.11.3"
     pg-connection-string: "npm:^2.3.0"
     supertest: "npm:^7.0.0"
-    testcontainers: "npm:^10.0.0"
+    testcontainers: "npm:^11.9.0"
     text-extensions: "npm:^2.4.0"
     uuid: "npm:^11.0.0"
     yn: "npm:^4.0.0"
@@ -20516,7 +20516,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@types/dockerode@npm:^3.3.35":
+"@types/dockerode@npm:^3.3.47":
   version: 3.3.47
   resolution: "@types/dockerode@npm:3.3.47"
   dependencies:
@@ -27965,7 +27965,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"debug@npm:4, debug@npm:^4, debug@npm:^4.0.0, debug@npm:^4.1.0, debug@npm:^4.1.1, debug@npm:^4.3.1, debug@npm:^4.3.2, debug@npm:^4.3.3, debug@npm:^4.3.4, debug@npm:^4.3.5, debug@npm:^4.3.6, debug@npm:^4.3.7, debug@npm:^4.4.0, debug@npm:^4.4.1":
+"debug@npm:4, debug@npm:^4, debug@npm:^4.0.0, debug@npm:^4.1.0, debug@npm:^4.1.1, debug@npm:^4.3.1, debug@npm:^4.3.2, debug@npm:^4.3.3, debug@npm:^4.3.4, debug@npm:^4.3.5, debug@npm:^4.3.6, debug@npm:^4.3.7, debug@npm:^4.4.0, debug@npm:^4.4.1, debug@npm:^4.4.3":
   version: 4.4.3
   resolution: "debug@npm:4.4.3"
   dependencies:
@@ -28562,12 +28562,12 @@ __metadata:
   languageName: node
   linkType: hard
 
-"docker-compose@npm:^0.24.8":
-  version: 0.24.8
-  resolution: "docker-compose@npm:0.24.8"
+"docker-compose@npm:^1.3.0":
+  version: 1.3.0
+  resolution: "docker-compose@npm:1.3.0"
   dependencies:
     yaml: "npm:^2.2.2"
-  checksum: 10/2b8526f9797a55c819ff2d7dcea57085b012b3a3d77bc2e1a6b45c3fc9e82196312f5298cbe8299966462454a5ac8f68814bb407736b4385e0d226a2a39e877a
+  checksum: 10/7c1b395fc104031eadef08cac0c028af11fd9e3084265dca9e17bc76ad27c2bfe96d8b74df258bb64b198f9e0f1792a7f2c63123cddc0d60bb776f211d54cfb2
   languageName: node
   linkType: hard
 
@@ -28583,7 +28583,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"dockerode@npm:^4.0.0, dockerode@npm:^4.0.5":
+"dockerode@npm:^4.0.0, dockerode@npm:^4.0.9":
   version: 4.0.9
   resolution: "dockerode@npm:4.0.9"
   dependencies:
@@ -47198,7 +47198,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"tar-fs@npm:^3.0.7, tar-fs@npm:^3.0.9":
+"tar-fs@npm:^3.0.9, tar-fs@npm:^3.1.1":
   version: 3.1.1
   resolution: "tar-fs@npm:3.1.1"
   dependencies:
@@ -47411,26 +47411,26 @@ __metadata:
   languageName: node
   linkType: hard
 
-"testcontainers@npm:^10.0.0":
-  version: 10.28.0
-  resolution: "testcontainers@npm:10.28.0"
+"testcontainers@npm:^11.9.0":
+  version: 11.9.0
+  resolution: "testcontainers@npm:11.9.0"
   dependencies:
     "@balena/dockerignore": "npm:^1.0.2"
-    "@types/dockerode": "npm:^3.3.35"
+    "@types/dockerode": "npm:^3.3.47"
     archiver: "npm:^7.0.1"
     async-lock: "npm:^1.4.1"
     byline: "npm:^5.0.0"
-    debug: "npm:^4.3.5"
-    docker-compose: "npm:^0.24.8"
-    dockerode: "npm:^4.0.5"
+    debug: "npm:^4.4.3"
+    docker-compose: "npm:^1.3.0"
+    dockerode: "npm:^4.0.9"
     get-port: "npm:^7.1.0"
     proper-lockfile: "npm:^4.1.2"
     properties-reader: "npm:^2.3.0"
     ssh-remote-port-forward: "npm:^1.0.4"
-    tar-fs: "npm:^3.0.7"
-    tmp: "npm:^0.2.3"
-    undici: "npm:^5.29.0"
-  checksum: 10/434d3677e10a114805420f2420831a8eae4091acdaf242787fb100a8755140af0e11eab3932cdb29267f0869af22d0b572532f72ee5450d60f63f3fed30d098c
+    tar-fs: "npm:^3.1.1"
+    tmp: "npm:^0.2.5"
+    undici: "npm:^7.16.0"
+  checksum: 10/974d729dfd2e511d8623f60629ddfb60446f6c0d0b9f41ac2db279fb94d89f6bc6bf7625734bff33c8d740886c3891cc29bda8f751edc46db6d42ca9212ba9fe
   languageName: node
   linkType: hard
 
@@ -47652,10 +47652,10 @@ __metadata:
   languageName: node
   linkType: hard
 
-"tmp@npm:^0.2.3":
-  version: 0.2.3
-  resolution: "tmp@npm:0.2.3"
-  checksum: 10/7b13696787f159c9754793a83aa79a24f1522d47b87462ddb57c18ee93ff26c74cbb2b8d9138f571d2e0e765c728fb2739863a672b280528512c6d83d511c6fa
+"tmp@npm:^0.2.5":
+  version: 0.2.5
+  resolution: "tmp@npm:0.2.5"
+  checksum: 10/dd4b78b32385eab4899d3ae296007b34482b035b6d73e1201c4a9aede40860e90997a1452c65a2d21aee73d53e93cd167d741c3db4015d90e63b6d568a93d7ec
   languageName: node
   linkType: hard
 
@@ -48603,7 +48603,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"undici@npm:^5.28.2, undici@npm:^5.29.0":
+"undici@npm:^5.28.2":
   version: 5.29.0
   resolution: "undici@npm:5.29.0"
   dependencies:
@@ -48612,10 +48612,10 @@ __metadata:
   languageName: node
   linkType: hard
 
-"undici@npm:^7.2.3":
-  version: 7.9.0
-  resolution: "undici@npm:7.9.0"
-  checksum: 10/fa92d3d9106612be566e79942174e00426c2e1f9a688fb86c8d1809c84a032c02fc57bd601d2051a45d94bbd312c50221644b58c8186c4f0d64f8ecc7f18b806
+"undici@npm:^7.16.0, undici@npm:^7.2.3":
+  version: 7.16.0
+  resolution: "undici@npm:7.16.0"
+  checksum: 10/2bb71672b23d3dc0f56f1b7fb6c936e4487a350db46eaafc03f2f9107f99cdf8e51ecdd32e589e2381ef47a64b6369cfb31f328b2c3ea663023aa47bc5258b9e
   languageName: node
   linkType: hard