diff --git a/cirq-google/cirq_google/api/v2/run_context.proto b/cirq-google/cirq_google/api/v2/run_context.proto
index a66a744a2c8..c826a0ad5ea 100644
--- a/cirq-google/cirq_google/api/v2/run_context.proto
+++ b/cirq-google/cirq_google/api/v2/run_context.proto
@@ -85,6 +85,13 @@ message SweepFunction {
     // "a": 1.0, "b": 3.0
     // Note: if one sweep is shorter, the others will be truncated.
     ZIP = 2;
+
+    // A zip product of parameter sweeps with length as the longest one.
+    //
+    // Suppose we zip_longest([sweep.points(a, [1, 2]), sweep.points(b, [3])]),
+    // the iterator will produce: {a: 1, b: 3} and {a: 2, b: 3}.
+    // The shorter sweeps will be filled by repeating their last value.
+    ZIP_LONGEST = 3;
   }
 
   FunctionType function_type = 1;
diff --git a/cirq-google/cirq_google/api/v2/run_context_pb2.py b/cirq-google/cirq_google/api/v2/run_context_pb2.py
index 5bc2c4ac6a9..145bb0aee9c 100644
--- a/cirq-google/cirq_google/api/v2/run_context_pb2.py
+++ b/cirq-google/cirq_google/api/v2/run_context_pb2.py
@@ -14,7 +14,7 @@
 from . import program_pb2 as cirq__google_dot_api_dot_v2_dot_program__pb2
 
 
-DESCRIPTOR = _descriptor_pool.Default().AddSerializedFile(b'\n$cirq_google/api/v2/run_context.proto\x12\x12\x63irq.google.api.v2\x1a cirq_google/api/v2/program.proto\"\x98\x01\n\nRunContext\x12<\n\x10parameter_sweeps\x18\x01 \x03(\x0b\x32\".cirq.google.api.v2.ParameterSweep\x12L\n\x1a\x64\x65vice_parameters_override\x18\x02 \x01(\x0b\x32(.cirq.google.api.v2.DeviceParametersDiff\"O\n\x0eParameterSweep\x12\x13\n\x0brepetitions\x18\x01 \x01(\x05\x12(\n\x05sweep\x18\x02 \x01(\x0b\x32\x19.cirq.google.api.v2.Sweep\"\x86\x01\n\x05Sweep\x12;\n\x0esweep_function\x18\x01 \x01(\x0b\x32!.cirq.google.api.v2.SweepFunctionH\x00\x12\x37\n\x0csingle_sweep\x18\x02 \x01(\x0b\x32\x1f.cirq.google.api.v2.SingleSweepH\x00\x42\x07\n\x05sweep\"\xc6\x01\n\rSweepFunction\x12\x45\n\rfunction_type\x18\x01 \x01(\x0e\x32..cirq.google.api.v2.SweepFunction.FunctionType\x12)\n\x06sweeps\x18\x02 \x03(\x0b\x32\x19.cirq.google.api.v2.Sweep\"C\n\x0c\x46unctionType\x12\x1d\n\x19\x46UNCTION_TYPE_UNSPECIFIED\x10\x00\x12\x0b\n\x07PRODUCT\x10\x01\x12\x07\n\x03ZIP\x10\x02\"W\n\x0f\x44\x65viceParameter\x12\x0c\n\x04path\x18\x01 \x03(\t\x12\x10\n\x03idx\x18\x02 \x01(\x03H\x00\x88\x01\x01\x12\x12\n\x05units\x18\x03 \x01(\tH\x01\x88\x01\x01\x42\x06\n\x04_idxB\x08\n\x06_units\"\xcf\x03\n\x14\x44\x65viceParametersDiff\x12\x46\n\x06groups\x18\x01 \x03(\x0b\x32\x36.cirq.google.api.v2.DeviceParametersDiff.ResourceGroup\x12>\n\x06params\x18\x02 \x03(\x0b\x32..cirq.google.api.v2.DeviceParametersDiff.Param\x12\x0c\n\x04strs\x18\x04 \x03(\t\x1a-\n\rResourceGroup\x12\x0e\n\x06parent\x18\x01 \x01(\x05\x12\x0c\n\x04name\x18\x02 \x01(\x05\x1a\x36\n\x0cGenericValue\x12\x17\n\x0ftype_descriptor\x18\x01 \x01(\t\x12\r\n\x05value\x18\x02 \x01(\x0c\x1a\xb9\x01\n\x05Param\x12\x16\n\x0eresource_group\x18\x01 \x01(\x05\x12\x0c\n\x04name\x18\x02 \x01(\x05\x12-\n\x05value\x18\x03 \x01(\x0b\x32\x1c.cirq.google.api.v2.ArgValueH\x00\x12N\n\rgeneric_value\x18\x04 \x01(\x0b\x32\x35.cirq.google.api.v2.DeviceParametersDiff.GenericValueH\x00\x42\x0b\n\tparam_val\"\xfc\x01\n\x0bSingleSweep\x12\x15\n\rparameter_key\x18\x01 \x01(\t\x12,\n\x06points\x18\x02 \x01(\x0b\x32\x1a.cirq.google.api.v2.PointsH\x00\x12\x30\n\x08linspace\x18\x03 \x01(\x0b\x32\x1c.cirq.google.api.v2.LinspaceH\x00\x12\x35\n\x0b\x63onst_value\x18\x05 \x01(\x0b\x32\x1e.cirq.google.api.v2.ConstValueH\x00\x12\x36\n\tparameter\x18\x04 \x01(\x0b\x32#.cirq.google.api.v2.DeviceParameterB\x07\n\x05sweep\"\x18\n\x06Points\x12\x0e\n\x06points\x18\x01 \x03(\x02\"G\n\x08Linspace\x12\x13\n\x0b\x66irst_point\x18\x01 \x01(\x02\x12\x12\n\nlast_point\x18\x02 \x01(\x02\x12\x12\n\nnum_points\x18\x03 \x01(\x03\"l\n\nConstValue\x12\x11\n\x07is_none\x18\x01 \x01(\x08H\x00\x12\x15\n\x0b\x66loat_value\x18\x02 \x01(\x02H\x00\x12\x13\n\tint_value\x18\x03 \x01(\x03H\x00\x12\x16\n\x0cstring_value\x18\x04 \x01(\tH\x00\x42\x07\n\x05valueB2\n\x1d\x63om.google.cirq.google.api.v2B\x0fRunContextProtoP\x01\x62\x06proto3')
+DESCRIPTOR = _descriptor_pool.Default().AddSerializedFile(b'\n$cirq_google/api/v2/run_context.proto\x12\x12\x63irq.google.api.v2\x1a cirq_google/api/v2/program.proto\"\x98\x01\n\nRunContext\x12<\n\x10parameter_sweeps\x18\x01 \x03(\x0b\x32\".cirq.google.api.v2.ParameterSweep\x12L\n\x1a\x64\x65vice_parameters_override\x18\x02 \x01(\x0b\x32(.cirq.google.api.v2.DeviceParametersDiff\"O\n\x0eParameterSweep\x12\x13\n\x0brepetitions\x18\x01 \x01(\x05\x12(\n\x05sweep\x18\x02 \x01(\x0b\x32\x19.cirq.google.api.v2.Sweep\"\x86\x01\n\x05Sweep\x12;\n\x0esweep_function\x18\x01 \x01(\x0b\x32!.cirq.google.api.v2.SweepFunctionH\x00\x12\x37\n\x0csingle_sweep\x18\x02 \x01(\x0b\x32\x1f.cirq.google.api.v2.SingleSweepH\x00\x42\x07\n\x05sweep\"\xd7\x01\n\rSweepFunction\x12\x45\n\rfunction_type\x18\x01 \x01(\x0e\x32..cirq.google.api.v2.SweepFunction.FunctionType\x12)\n\x06sweeps\x18\x02 \x03(\x0b\x32\x19.cirq.google.api.v2.Sweep\"T\n\x0c\x46unctionType\x12\x1d\n\x19\x46UNCTION_TYPE_UNSPECIFIED\x10\x00\x12\x0b\n\x07PRODUCT\x10\x01\x12\x07\n\x03ZIP\x10\x02\x12\x0f\n\x0bZIP_LONGEST\x10\x03\"W\n\x0f\x44\x65viceParameter\x12\x0c\n\x04path\x18\x01 \x03(\t\x12\x10\n\x03idx\x18\x02 \x01(\x03H\x00\x88\x01\x01\x12\x12\n\x05units\x18\x03 \x01(\tH\x01\x88\x01\x01\x42\x06\n\x04_idxB\x08\n\x06_units\"\xcf\x03\n\x14\x44\x65viceParametersDiff\x12\x46\n\x06groups\x18\x01 \x03(\x0b\x32\x36.cirq.google.api.v2.DeviceParametersDiff.ResourceGroup\x12>\n\x06params\x18\x02 \x03(\x0b\x32..cirq.google.api.v2.DeviceParametersDiff.Param\x12\x0c\n\x04strs\x18\x04 \x03(\t\x1a-\n\rResourceGroup\x12\x0e\n\x06parent\x18\x01 \x01(\x05\x12\x0c\n\x04name\x18\x02 \x01(\x05\x1a\x36\n\x0cGenericValue\x12\x17\n\x0ftype_descriptor\x18\x01 \x01(\t\x12\r\n\x05value\x18\x02 \x01(\x0c\x1a\xb9\x01\n\x05Param\x12\x16\n\x0eresource_group\x18\x01 \x01(\x05\x12\x0c\n\x04name\x18\x02 \x01(\x05\x12-\n\x05value\x18\x03 \x01(\x0b\x32\x1c.cirq.google.api.v2.ArgValueH\x00\x12N\n\rgeneric_value\x18\x04 \x01(\x0b\x32\x35.cirq.google.api.v2.DeviceParametersDiff.GenericValueH\x00\x42\x0b\n\tparam_val\"\xfc\x01\n\x0bSingleSweep\x12\x15\n\rparameter_key\x18\x01 \x01(\t\x12,\n\x06points\x18\x02 \x01(\x0b\x32\x1a.cirq.google.api.v2.PointsH\x00\x12\x30\n\x08linspace\x18\x03 \x01(\x0b\x32\x1c.cirq.google.api.v2.LinspaceH\x00\x12\x35\n\x0b\x63onst_value\x18\x05 \x01(\x0b\x32\x1e.cirq.google.api.v2.ConstValueH\x00\x12\x36\n\tparameter\x18\x04 \x01(\x0b\x32#.cirq.google.api.v2.DeviceParameterB\x07\n\x05sweep\"\x18\n\x06Points\x12\x0e\n\x06points\x18\x01 \x03(\x02\"G\n\x08Linspace\x12\x13\n\x0b\x66irst_point\x18\x01 \x01(\x02\x12\x12\n\nlast_point\x18\x02 \x01(\x02\x12\x12\n\nnum_points\x18\x03 \x01(\x03\"l\n\nConstValue\x12\x11\n\x07is_none\x18\x01 \x01(\x08H\x00\x12\x15\n\x0b\x66loat_value\x18\x02 \x01(\x02H\x00\x12\x13\n\tint_value\x18\x03 \x01(\x03H\x00\x12\x16\n\x0cstring_value\x18\x04 \x01(\tH\x00\x42\x07\n\x05valueB2\n\x1d\x63om.google.cirq.google.api.v2B\x0fRunContextProtoP\x01\x62\x06proto3')
 
 _globals = globals()
 _builder.BuildMessageAndEnumDescriptors(DESCRIPTOR, _globals)
@@ -29,25 +29,25 @@
   _globals['_SWEEP']._serialized_start=331
   _globals['_SWEEP']._serialized_end=465
   _globals['_SWEEPFUNCTION']._serialized_start=468
-  _globals['_SWEEPFUNCTION']._serialized_end=666
+  _globals['_SWEEPFUNCTION']._serialized_end=683
   _globals['_SWEEPFUNCTION_FUNCTIONTYPE']._serialized_start=599
-  _globals['_SWEEPFUNCTION_FUNCTIONTYPE']._serialized_end=666
-  _globals['_DEVICEPARAMETER']._serialized_start=668
-  _globals['_DEVICEPARAMETER']._serialized_end=755
-  _globals['_DEVICEPARAMETERSDIFF']._serialized_start=758
-  _globals['_DEVICEPARAMETERSDIFF']._serialized_end=1221
-  _globals['_DEVICEPARAMETERSDIFF_RESOURCEGROUP']._serialized_start=932
-  _globals['_DEVICEPARAMETERSDIFF_RESOURCEGROUP']._serialized_end=977
-  _globals['_DEVICEPARAMETERSDIFF_GENERICVALUE']._serialized_start=979
-  _globals['_DEVICEPARAMETERSDIFF_GENERICVALUE']._serialized_end=1033
-  _globals['_DEVICEPARAMETERSDIFF_PARAM']._serialized_start=1036
-  _globals['_DEVICEPARAMETERSDIFF_PARAM']._serialized_end=1221
-  _globals['_SINGLESWEEP']._serialized_start=1224
-  _globals['_SINGLESWEEP']._serialized_end=1476
-  _globals['_POINTS']._serialized_start=1478
-  _globals['_POINTS']._serialized_end=1502
-  _globals['_LINSPACE']._serialized_start=1504
-  _globals['_LINSPACE']._serialized_end=1575
-  _globals['_CONSTVALUE']._serialized_start=1577
-  _globals['_CONSTVALUE']._serialized_end=1685
+  _globals['_SWEEPFUNCTION_FUNCTIONTYPE']._serialized_end=683
+  _globals['_DEVICEPARAMETER']._serialized_start=685
+  _globals['_DEVICEPARAMETER']._serialized_end=772
+  _globals['_DEVICEPARAMETERSDIFF']._serialized_start=775
+  _globals['_DEVICEPARAMETERSDIFF']._serialized_end=1238
+  _globals['_DEVICEPARAMETERSDIFF_RESOURCEGROUP']._serialized_start=949
+  _globals['_DEVICEPARAMETERSDIFF_RESOURCEGROUP']._serialized_end=994
+  _globals['_DEVICEPARAMETERSDIFF_GENERICVALUE']._serialized_start=996
+  _globals['_DEVICEPARAMETERSDIFF_GENERICVALUE']._serialized_end=1050
+  _globals['_DEVICEPARAMETERSDIFF_PARAM']._serialized_start=1053
+  _globals['_DEVICEPARAMETERSDIFF_PARAM']._serialized_end=1238
+  _globals['_SINGLESWEEP']._serialized_start=1241
+  _globals['_SINGLESWEEP']._serialized_end=1493
+  _globals['_POINTS']._serialized_start=1495
+  _globals['_POINTS']._serialized_end=1519
+  _globals['_LINSPACE']._serialized_start=1521
+  _globals['_LINSPACE']._serialized_end=1592
+  _globals['_CONSTVALUE']._serialized_start=1594
+  _globals['_CONSTVALUE']._serialized_end=1702
 # @@protoc_insertion_point(module_scope)
diff --git a/cirq-google/cirq_google/api/v2/run_context_pb2.pyi b/cirq-google/cirq_google/api/v2/run_context_pb2.pyi
index 896804c19d1..99618d75629 100644
--- a/cirq-google/cirq_google/api/v2/run_context_pb2.pyi
+++ b/cirq-google/cirq_google/api/v2/run_context_pb2.pyi
@@ -156,6 +156,13 @@ class SweepFunction(google.protobuf.message.Message):
         "a": 1.0, "b": 3.0
         Note: if one sweep is shorter, the others will be truncated.
         """
+        ZIP_LONGEST: SweepFunction._FunctionType.ValueType  # 3
+        """A zip product of parameter sweeps with length as the longest one.
+
+        Suppose we zip_longest([sweep.points(a, [1, 2]), sweep.points(b, [3])]),
+        the iterator will produce: {a: 1, b: 3} and {a: 2, b: 3}.
+        The shorter sweeps will be filled by repeating their last value.
+        """
 
     class FunctionType(_FunctionType, metaclass=_FunctionTypeEnumTypeWrapper):
         """The type of sweep function."""
@@ -193,6 +200,13 @@ class SweepFunction(google.protobuf.message.Message):
     "a": 1.0, "b": 3.0
     Note: if one sweep is shorter, the others will be truncated.
     """
+    ZIP_LONGEST: SweepFunction.FunctionType.ValueType  # 3
+    """A zip product of parameter sweeps with length as the longest one.
+
+    Suppose we zip_longest([sweep.points(a, [1, 2]), sweep.points(b, [3])]),
+    the iterator will produce: {a: 1, b: 3} and {a: 2, b: 3}.
+    The shorter sweeps will be filled by repeating their last value.
+    """
 
     FUNCTION_TYPE_FIELD_NUMBER: builtins.int
     SWEEPS_FIELD_NUMBER: builtins.int
diff --git a/cirq-google/cirq_google/api/v2/sweeps.py b/cirq-google/cirq_google/api/v2/sweeps.py
index d3b71fe718a..d2a39b926d8 100644
--- a/cirq-google/cirq_google/api/v2/sweeps.py
+++ b/cirq-google/cirq_google/api/v2/sweeps.py
@@ -73,6 +73,10 @@ def sweep_to_proto(
         out.sweep_function.function_type = run_context_pb2.SweepFunction.PRODUCT
         for factor in sweep.factors:
             sweep_to_proto(factor, out=out.sweep_function.sweeps.add())
+    elif isinstance(sweep, cirq.ZipLongest):
+        out.sweep_function.function_type = run_context_pb2.SweepFunction.ZIP_LONGEST
+        for s in sweep.sweeps:
+            sweep_to_proto(s, out=out.sweep_function.sweeps.add())
     elif isinstance(sweep, cirq.Zip):
         out.sweep_function.function_type = run_context_pb2.SweepFunction.ZIP
         for s in sweep.sweeps:
@@ -129,6 +133,8 @@ def sweep_from_proto(msg: run_context_pb2.Sweep) -> cirq.Sweep:
             return cirq.Product(*factors)
         if func_type == run_context_pb2.SweepFunction.ZIP:
             return cirq.Zip(*factors)
+        if func_type == run_context_pb2.SweepFunction.ZIP_LONGEST:
+            return cirq.ZipLongest(*factors)
 
         raise ValueError(f'invalid sweep function type: {func_type}')
     if which == 'single_sweep':
diff --git a/cirq-google/cirq_google/api/v2/sweeps_test.py b/cirq-google/cirq_google/api/v2/sweeps_test.py
index 0e16ecffe66..0535e60f5c4 100644
--- a/cirq-google/cirq_google/api/v2/sweeps_test.py
+++ b/cirq-google/cirq_google/api/v2/sweeps_test.py
@@ -68,6 +68,7 @@ def _values(self) -> Iterator[float]:
                 + (cirq.Points('g', [1, 2]) * cirq.Points('h', [-1, 0, 1]))
             )
         ),
+        cirq.ZipLongest(cirq.Points('a', [1.0, 2.0, 3.0]), cirq.Points('b', [1.0])),
         # Sweep with constant. Type ignore is because cirq.Points type annotated with floats.
         cirq.Points('a', [None]),  # type: ignore[list-item]
         cirq.Points('a', [None]) * cirq.Points('b', [1, 2, 3]),  # type: ignore[list-item]