diff --git a/http4s-backend/src/main/scala/sttp/client4/http4s/Http4sBackendBase.scala b/http4s-backend/src/main/scala/sttp/client4/http4s/Http4sBackendBase.scala
index 5d302da95b..6c37dcb3da 100644
--- a/http4s-backend/src/main/scala/sttp/client4/http4s/Http4sBackendBase.scala
+++ b/http4s-backend/src/main/scala/sttp/client4/http4s/Http4sBackendBase.scala
@@ -111,10 +111,15 @@ private[http4s] abstract class Http4sBackendBase[F[_]](implicit protected val as
             }
             .recoverWith { case t: Throwable => responseVar.complete(Left(t)).as(()) }
 
-          sendRequest.start >> responseVar.get.flatMap {
-            case Left(t)  => implicitly[cats.ApplicativeError[F, Throwable]].raiseError(t)
-            case Right(r) => r.pure[F]
-          }
+          // uncancelable, so that a cancel between starting the fiber and installing onCancel doesn't leak the fiber
+          asyncF
+            .uncancelable { poll =>
+              sendRequest.start.flatMap(fiber => poll(responseVar.get).onCancel(fiber.cancel))
+            }
+            .flatMap {
+              case Left(t)  => asyncF.raiseError[Response[T]](t)
+              case Right(r) => r.pure[F]
+            }
         }
       }
     }
diff --git a/http4s-backend/src/test/scalajvm/sttp/client4/http4s/Http4sBackendCancellationTest.scala b/http4s-backend/src/test/scalajvm/sttp/client4/http4s/Http4sBackendCancellationTest.scala
new file mode 100644
index 0000000000..3c4bd196fa
--- /dev/null
+++ b/http4s-backend/src/test/scalajvm/sttp/client4/http4s/Http4sBackendCancellationTest.scala
@@ -0,0 +1,38 @@
+package sttp.client4.http4s
+
+import cats.effect.{Deferred, IO}
+import cats.effect.unsafe.IORuntime
+import org.http4s.{Response => Http4sResponse}
+import org.http4s.client.Client
+import org.scalatest.flatspec.AsyncFlatSpec
+import org.scalatest.matchers.should.Matchers
+import sttp.client4._
+
+import scala.concurrent.duration._
+
+class Http4sBackendCancellationTest extends AsyncFlatSpec with Matchers {
+
+  implicit val ioRuntime: IORuntime = IORuntime.global
+
+  it should "cancel the underlying request fiber when the caller cancels" in {
+    val test = for {
+      started <- Deferred[IO, Unit]
+      cancelled <- Deferred[IO, Unit]
+      // A client whose request never completes, but records when it starts and when it is cancelled
+      client = Client[IO] { _ =>
+        (started.complete(()) >> IO.never[Http4sResponse[IO]])
+          .onCancel(cancelled.complete(()).void)
+          .toResource
+      }
+      backend = Http4sBackend.usingClient[IO](client)
+      req = basicRequest.get(uri"http://localhost/test").response(asString)
+      fiber <- req.send(backend).start
+      _ <- started.get.timeout(3.seconds)
+      _ <- fiber.cancel
+      // If the fiber was properly cancelled, onCancel will have signalled
+      _ <- cancelled.get.timeout(3.seconds)
+    } yield succeed
+
+    test.unsafeToFuture()
+  }
+}