diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index c186ab1434ef..0a9e3856f217 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -2202,13 +2202,28 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
                 # will be reported for types incompatible with __contains__().
                 # See testCustomContainsCheckStrictEquality for an example.
                 cont_type = self.chk.analyze_container_item_type(right_type)
+                iter_type = right_type
+                changed_expr = right
+                proper_right_type = get_proper_type(right_type)
+                if (local_errors.is_errors() and
+                        isinstance(proper_right_type, UnionType)):
+                    typs = []  # type: List[Type]
+                    for item in proper_right_type.relevant_items():
+                        temp_errors = self.msg.copy()
+                        temp_errors.disable_count = 0
+                        self.check_method_call_by_name(
+                            '__contains__', item, [left], [ARG_POS], e, temp_errors)
+                        if temp_errors.is_errors():
+                            typs.append(item)
+                    iter_type = UnionType.make_union(typs)
+                    changed_expr = TempNode(iter_type)
                 if isinstance(right_type, PartialType):
                     # We don't really know if this is an error or not, so just shut up.
                     pass
                 elif (local_errors.is_errors() and
                     # is_valid_var_arg is True for any Iterable
-                        self.is_valid_var_arg(right_type)):
-                    _, itertype = self.chk.analyze_iterable_item_type(right)
+                        self.is_valid_var_arg(iter_type)):
+                    _, itertype = self.chk.analyze_iterable_item_type(changed_expr)
                     method_type = CallableType(
                         [left_type],
                         [nodes.ARG_POS],
diff --git a/test-data/unit/check-expressions.test b/test-data/unit/check-expressions.test
index 4eb52be6f8bd..d0127d9c916b 100644
--- a/test-data/unit/check-expressions.test
+++ b/test-data/unit/check-expressions.test
@@ -406,6 +406,15 @@ class D(Iterable[A]):
     def __iter__(self) -> Iterator[A]: pass
 [builtins fixtures/bool.pyi]
 
+[case testInOperatorUnion]
+from typing import Iterable, Union, Container
+u: Union[Iterable[str], Container[str], Iterable[str]]
+'x' in u
+
+w: Union[Container[str], int]
+'x' in w   # E: Unsupported right operand type for in ("Union[Container[str], int]")
+[typing fixtures/typing-full.pyi]
+
 [case testNotInOperator]
 from typing import Iterator, Iterable, Any
 a, b, c, d, e = None, None, None, None, None # type: (A, B, bool, D, Any)