diff --git a/src/allmydata/storage/http_client.py b/src/allmydata/storage/http_client.py
index 475aa2330..d83ecbdff 100644
--- a/src/allmydata/storage/http_client.py
+++ b/src/allmydata/storage/http_client.py
@@ -161,7 +161,7 @@ class StorageClientImmutables(object):
     APIs for interacting with immutables.
     """
 
-    def __init__(self, client):  # type: (StorageClient) -> None
+    def __init__(self, client: StorageClient):
         self._client = client
 
     @inlineCallbacks
@@ -208,6 +208,27 @@ class StorageClientImmutables(object):
             )
         )
 
+    @inlineCallbacks
+    def abort_upload(
+        self, storage_index: bytes, share_number: int, upload_secret: bytes
+    ) -> Deferred[None]:
+        """Abort the upload."""
+        url = self._client.relative_url(
+            "/v1/immutable/{}/{}/abort".format(_encode_si(storage_index), share_number)
+        )
+        response = yield self._client.request(
+            "PUT",
+            url,
+            upload_secret=upload_secret,
+        )
+
+        if response.code == http.OK:
+            return
+        else:
+            raise ClientException(
+                response.code,
+            )
+
     @inlineCallbacks
     def write_share_chunk(
         self, storage_index, share_number, upload_secret, offset, data
diff --git a/src/allmydata/storage/http_server.py b/src/allmydata/storage/http_server.py
index d0653a97c..1acf08a81 100644
--- a/src/allmydata/storage/http_server.py
+++ b/src/allmydata/storage/http_server.py
@@ -303,6 +303,33 @@ class HTTPServer(object):
             },
         )
 
+    @_authorized_route(
+        _app,
+        {Secrets.UPLOAD},
+        "/v1/immutable/<storage_index:storage_index>/<int(signed=False):share_number>/abort",
+        methods=["PUT"],
+    )
+    def abort_share_upload(self, request, authorization, storage_index, share_number):
+        """Abort an in-progress immutable share upload."""
+        try:
+            bucket = self._uploads.get_write_bucket(
+                storage_index, share_number, authorization[Secrets.UPLOAD]
+            )
+        except _HTTPError:
+            # TODO 3877 If 404, check if this was already uploaded, in which case return 405
+            # TODO 3877 write tests for 404 cases?
+            raise
+
+        # TODO 3877 test for checking upload secret
+
+        # Abort the upload:
+        bucket.abort()
+        # Stop tracking the bucket, so we can create a new one later if a
+        # client requests it:
+        self._uploads.remove_write_bucket(storage_index, share_number)
+
+        return b""
+
     @_authorized_route(
         _app,
         {Secrets.UPLOAD},
diff --git a/src/allmydata/storage_client.py b/src/allmydata/storage_client.py
index c2e26b4b6..9e47ba3ff 100644
--- a/src/allmydata/storage_client.py
+++ b/src/allmydata/storage_client.py
@@ -1059,7 +1059,8 @@ class _HTTPBucketWriter(object):
     finished = attr.ib(type=bool, default=False)
 
     def abort(self):
-        pass  # TODO in later ticket
+        return self.client.abort_upload(self.storage_index, self.share_number,
+                                        self.upload_secret)
 
     @defer.inlineCallbacks
     def write(self, offset, data):
diff --git a/src/allmydata/test/test_istorageserver.py b/src/allmydata/test/test_istorageserver.py
index 95261ddb2..668eeecc5 100644
--- a/src/allmydata/test/test_istorageserver.py
+++ b/src/allmydata/test/test_istorageserver.py
@@ -176,8 +176,9 @@ class IStorageServerImmutableAPIsTestsMixin(object):
             canary=Referenceable(),
         )
 
-        # Bucket 1 is fully written in one go.
-        yield allocated[0].callRemote("write", 0, b"1" * 1024)
+        # Bucket 1 get some data written (but not all, or HTTP implicitly
+        # finishes the upload)
+        yield allocated[0].callRemote("write", 0, b"1" * 1023)
 
         # Disconnect or abort, depending on the test:
         yield abort_or_disconnect(allocated[0])
@@ -1156,7 +1157,6 @@ class HTTPImmutableAPIsTests(
 
     # These will start passing in future PRs as HTTP protocol is implemented.
     SKIP_TESTS = {
-        "test_abort",
         "test_add_lease_renewal",
         "test_add_new_lease",
         "test_advise_corrupt_share",