diff --git a/CHANGES/883.bugfix b/CHANGES/883.bugfix
new file mode 100644
index 00000000..464c38b4
--- /dev/null
+++ b/CHANGES/883.bugfix
@@ -0,0 +1 @@
+Fix URL.build with port="" creating invalid URLs
diff --git a/tests/test_url_build.py b/tests/test_url_build.py
index 51969fa8..03478990 100644
--- a/tests/test_url_build.py
+++ b/tests/test_url_build.py
@@ -39,6 +39,9 @@ def test_build_with_port():
     u = URL.build(scheme="http", host="127.0.0.1", port=8000)
     assert str(u) == "http://127.0.0.1:8000"
 
+    u = URL.build(scheme="http", host="127.0.0.1", port="", path="/v1")
+    assert str(u) == "http://127.0.0.1/v1", str(u)
+
 
 def test_build_with_user():
     u = URL.build(scheme="http", host="127.0.0.1", user="foo")
diff --git a/yarl/_url.py b/yarl/_url.py
index c8f2acb3..685313cf 100644
--- a/yarl/_url.py
+++ b/yarl/_url.py
@@ -233,6 +233,8 @@ def build(
             raise ValueError(
                 'Can\'t mix "authority" with "user", "password", "host" or "port".'
             )
+        if isinstance(port, str) and port == "":
+            port = None
         if port and not host:
             raise ValueError('Can\'t build URL with "port" but without "host".')
         if query and query_string: