diff --git a/test/js/bun/s3/s3.test.ts b/test/js/bun/s3/s3.test.ts
index 2c77421b01..e8a4fde3b3 100644
--- a/test/js/bun/s3/s3.test.ts
+++ b/test/js/bun/s3/s3.test.ts
@@ -455,6 +455,83 @@ for (let credentials of allCredentials) {
               }
             });
 
+            it("should enforce contentLength restrictions on S3Client presigned URLs", async () => {
+              const testContent = "Test data for S3Client";  // 23 bytes
+              const contentLength = testContent.length;
+              const uploadFilename = bucketInName ? `${S3Bucket}/${randomUUID()}-s3client` : `${randomUUID()}-s3client`;
+              
+              // Test 1: Upload exactly contentLength bytes - should succeed
+              {
+                const presignedUrl = bucket.presign(uploadFilename, {
+                  method: "PUT",
+                  expiresIn: 3600,
+                  contentLength: contentLength,
+                });
+                
+                expect(presignedUrl.includes(`Content-Length=${contentLength}`)).toBe(true);
+                
+                const response = await fetch(presignedUrl, {
+                  method: "PUT",
+                  body: testContent,  // Exactly 23 bytes
+                  headers: {
+                    "Content-Type": "text/plain",
+                  },
+                });
+                
+                expect(response.status).toBe(200);
+                
+                // Verify the content was uploaded correctly
+                const file = bucket.file(uploadFilename, options);
+                const downloaded = await file.text();
+                expect(downloaded).toBe(testContent);
+                
+                // Clean up
+                await file.unlink();
+              }
+              
+              // Test 2: Upload less than contentLength - should fail
+              {
+                const presignedUrl = bucket.presign(uploadFilename + "-less", {
+                  method: "PUT", 
+                  expiresIn: 3600,
+                  contentLength: contentLength,
+                });
+                
+                const shortContent = "Short";  // Only 5 bytes, less than 23
+                const response = await fetch(presignedUrl, {
+                  method: "PUT",
+                  body: shortContent,
+                  headers: {
+                    "Content-Type": "text/plain",
+                  },
+                });
+                
+                // Should fail with 400 Bad Request due to content length mismatch
+                expect([400, 403]).toContain(response.status);
+              }
+              
+              // Test 3: Upload more than contentLength - should fail  
+              {
+                const presignedUrl = bucket.presign(uploadFilename + "-more", {
+                  method: "PUT",
+                  expiresIn: 3600,
+                  contentLength: contentLength,
+                });
+                
+                const longContent = "This content is definitely much longer than the expected 23 bytes and should cause a failure";
+                const response = await fetch(presignedUrl, {
+                  method: "PUT",
+                  body: longContent,
+                  headers: {
+                    "Content-Type": "text/plain",
+                  },
+                });
+                
+                // Should fail with 400 Bad Request due to content length mismatch
+                expect([400, 403]).toContain(response.status);
+              }
+            });
+
             it("should be able to upload large files using bucket.write + readable Request", async () => {
               {
                 await bucket.write(
@@ -726,6 +803,117 @@ for (let credentials of allCredentials) {
               }
             });
 
+            it("should enforce contentLength restrictions on PUT presigned URLs", async () => {
+              const testContent = "Hello, Bun!";  // 12 bytes
+              const contentLength = testContent.length;
+              const uploadFilename = tmp_filename + "-contentlength-test";
+              
+              // Test 1: Upload exactly contentLength bytes - should succeed
+              {
+                const s3file = s3(uploadFilename, options);
+                const presignedUrl = s3file.presign({
+                  method: "PUT",
+                  expiresIn: 3600,
+                  contentLength: contentLength,
+                });
+                
+                expect(presignedUrl.includes(`Content-Length=${contentLength}`)).toBe(true);
+                
+                const response = await fetch(presignedUrl, {
+                  method: "PUT",
+                  body: testContent,  // Exactly 12 bytes
+                  headers: {
+                    "Content-Type": "text/plain",
+                  },
+                });
+                
+                expect(response.status).toBe(200);
+                
+                // Verify the content was uploaded correctly
+                const downloaded = await s3file.text();
+                expect(downloaded).toBe(testContent);
+                
+                // Clean up
+                await s3file.unlink();
+              }
+              
+              // Test 2: Upload less than contentLength - should fail
+              {
+                const s3file = s3(uploadFilename + "-less", options);
+                const presignedUrl = s3file.presign({
+                  method: "PUT", 
+                  expiresIn: 3600,
+                  contentLength: contentLength,
+                });
+                
+                const shortContent = "Short";  // Only 5 bytes, less than 12
+                const response = await fetch(presignedUrl, {
+                  method: "PUT",
+                  body: shortContent,
+                  headers: {
+                    "Content-Type": "text/plain",
+                  },
+                });
+                
+                // Should fail with 400 Bad Request due to content length mismatch
+                expect([400, 403]).toContain(response.status);
+              }
+              
+              // Test 3: Upload more than contentLength - should fail  
+              {
+                const s3file = s3(uploadFilename + "-more", options);
+                const presignedUrl = s3file.presign({
+                  method: "PUT",
+                  expiresIn: 3600,
+                  contentLength: contentLength,
+                });
+                
+                const longContent = "This is a much longer content than expected";  // Much more than 12 bytes
+                const response = await fetch(presignedUrl, {
+                  method: "PUT",
+                  body: longContent,
+                  headers: {
+                    "Content-Type": "text/plain",
+                  },
+                });
+                
+                // Should fail with 400 Bad Request due to content length mismatch
+                expect([400, 403]).toContain(response.status);
+              }
+            });
+
+            it("should work with ContentLength (AWS SDK style) restrictions", async () => {
+              const testData = Buffer.alloc(100, 'x'); // Exactly 100 bytes
+              const uploadFilename = tmp_filename + "-aws-style";
+              
+              // Test with AWS SDK style "ContentLength" 
+              const s3file = s3(uploadFilename, options);
+              const presignedUrl = s3file.presign({
+                method: "PUT",
+                expiresIn: 3600,
+                ContentLength: 100,  // AWS SDK style (PascalCase)
+              });
+              
+              expect(presignedUrl.includes("Content-Length=100")).toBe(true);
+              
+              const response = await fetch(presignedUrl, {
+                method: "PUT",
+                body: testData,  // Exactly 100 bytes
+                headers: {
+                  "Content-Type": "application/octet-stream",
+                },
+              });
+              
+              expect(response.status).toBe(200);
+              
+              // Verify upload
+              const stat = await s3file.stat();
+              expect(stat.size).toBe(100);
+              
+              // Clean up
+              await s3file.unlink();
+            });
+
             it("should be able to upload large files in one go using Bun.write", async () => {
               {
                 const s3file = s3(tmp_filename, options);
diff --git a/test/regression/issue/s3-content-length.test.ts b/test/regression/issue/s3-content-length.test.ts
index a6f1ab7687..81c7329efd 100644
--- a/test/regression/issue/s3-content-length.test.ts
+++ b/test/regression/issue/s3-content-length.test.ts
@@ -1,7 +1,7 @@
 import { S3Client } from "bun";
 import { describe, expect, it } from "bun:test";
 
-describe("S3 contentLength option in presign", () => {
+describe("S3 contentLength option in presign (Issue #18240)", () => {
   const s3Client = new S3Client({
     accessKeyId: "test-key",
     secretAccessKey: "test-secret",
@@ -46,4 +46,45 @@ describe("S3 contentLength option in presign", () => {
     expect(url.includes("Content-Length=")).toBe(false);
     expect(url.includes("X-Amz-Expires=3600")).toBe(true);
   });
+
+  it("should validate contentLength is positive", () => {
+    expect(() => {
+      s3Client.presign("test/abc", {
+        expiresIn: 3600,
+        method: "PUT",
+        contentLength: -1, // Invalid negative value
+      });
+    }).toThrow();
+  });
+
+  it("should validate ContentLength is positive", () => {
+    expect(() => {
+      s3Client.presign("test/abc", {
+        expiresIn: 3600,
+        method: "PUT",
+        ContentLength: -100, // Invalid negative value
+      });
+    }).toThrow();
+  });
+
+  it("should match the exact use case from issue #18240", () => {
+    // This is the exact code snippet from the GitHub issue
+    const url = s3Client.presign('test/abc', {
+      expiresIn: 3600, // 1 hour
+      method: 'PUT',
+      ContentLength: 200 // THIS IS NOW WORKING
+    });
+
+    expect(url).toBeDefined();
+    expect(typeof url).toBe("string");
+    expect(url.includes("Content-Length=200")).toBe(true);
+    
+    // Verify other required AWS S3 signature components are present
+    expect(url.includes("X-Amz-Expires=3600")).toBe(true);
+    expect(url.includes("X-Amz-Algorithm=AWS4-HMAC-SHA256")).toBe(true);
+    expect(url.includes("X-Amz-Credential")).toBe(true);
+    expect(url.includes("X-Amz-Date")).toBe(true);
+    expect(url.includes("X-Amz-SignedHeaders")).toBe(true);
+    expect(url.includes("X-Amz-Signature")).toBe(true);
+  });
 });
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