diff --git a/ax/modelbridge/transforms/map_key_to_float.py b/ax/modelbridge/transforms/map_key_to_float.py
new file mode 100644
index 00000000000..1ec645aff51
--- /dev/null
+++ b/ax/modelbridge/transforms/map_key_to_float.py
@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# pyre-strict
+
+from typing import Any, Optional, TYPE_CHECKING
+
+from ax.core.map_metric import MapMetric
+from ax.core.observation import Observation, ObservationFeatures
+from ax.core.search_space import SearchSpace
+from ax.modelbridge.transforms.metadata_to_range import MetadataToFloat
+from ax.models.types import TConfig
+from pyre_extensions import assert_is_instance
+
+if TYPE_CHECKING:
+    # import as module to make sphinx-autodoc-typehints happy
+    from ax import modelbridge as modelbridge_module  # noqa F401
+
+
+class MapKeyToFloat(MetadataToFloat):
+    DEFAULT_LOG_SCALE: bool = True
+    DEFAULT_MAP_KEY: str = MapMetric.map_key_info.key
+
+    def __init__(
+        self,
+        search_space: SearchSpace | None = None,
+        observations: list[Observation] | None = None,
+        modelbridge: Optional["modelbridge_module.base.ModelBridge"] = None,
+        config: TConfig | None = None,
+    ) -> None:
+        config = config or {}
+        self.parameters: dict[str, dict[str, Any]] = assert_is_instance(
+            config.setdefault("parameters", {}), dict
+        )
+        # TODO[tiao]: raise warning if `DEFAULT_MAP_KEY` is already in keys(?)
+        self.parameters.setdefault(self.DEFAULT_MAP_KEY, {})
+        super().__init__(
+            search_space=search_space,
+            observations=observations,
+            modelbridge=modelbridge,
+            config=config,
+        )
+
+    def _transform_observation_feature(self, obsf: ObservationFeatures) -> None:
+        if not obsf.parameters:
+            for p in self._parameter_list:
+                # TODO[tiao]: can we use be p.target_value?
+                # (not its original intended use but could be advantageous)
+                obsf.parameters[p.name] = p.upper
+            return
+        super()._transform_observation_feature(obsf)
diff --git a/ax/modelbridge/transforms/tests/test_map_key_to_float_transform.py b/ax/modelbridge/transforms/tests/test_map_key_to_float_transform.py
new file mode 100644
index 00000000000..c1ddb8c1d34
--- /dev/null
+++ b/ax/modelbridge/transforms/tests/test_map_key_to_float_transform.py
@@ -0,0 +1,174 @@
+#!/usr/bin/env python3
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# pyre-strict
+
+from copy import deepcopy
+from typing import Iterator
+
+import numpy as np
+from ax.core.observation import Observation, ObservationData, ObservationFeatures
+from ax.core.parameter import ParameterType, RangeParameter
+from ax.core.search_space import SearchSpace
+from ax.modelbridge.transforms.map_key_to_float import MapKeyToFloat
+from ax.utils.common.testutils import TestCase
+from pyre_extensions import assert_is_instance
+
+
+WIDTHS = [2.0, 4.0, 8.0]
+HEIGHTS = [4.0, 2.0, 8.0]
+STEPS_ENDS = [1, 5, 3]
+
+
+def _enumerate() -> Iterator[tuple[int, float, float, float]]:
+    yield from (
+        (trial_index, width, height, float(i + 1))
+        for trial_index, (width, height, steps_end) in enumerate(
+            zip(WIDTHS, HEIGHTS, STEPS_ENDS)
+        )
+        for i in range(steps_end)
+    )
+
+
+class MapKeyToFloatTransformTest(TestCase):
+    def setUp(self) -> None:
+        super().setUp()
+
+        self.search_space = SearchSpace(
+            parameters=[
+                RangeParameter(
+                    name="width",
+                    parameter_type=ParameterType.FLOAT,
+                    lower=1,
+                    upper=20,
+                ),
+                RangeParameter(
+                    name="height",
+                    parameter_type=ParameterType.FLOAT,
+                    lower=1,
+                    upper=20,
+                ),
+            ]
+        )
+
+        self.observations = []
+        for trial_index, width, height, steps in _enumerate():
+            obs_feat = ObservationFeatures(
+                trial_index=trial_index,
+                parameters={"width": width, "height": height},
+                metadata={
+                    "foo": 42,
+                    MapKeyToFloat.DEFAULT_MAP_KEY: steps,
+                },
+            )
+            obs_data = ObservationData(
+                metric_names=[], means=np.array([]), covariance=np.empty((0, 0))
+            )
+            self.observations.append(Observation(features=obs_feat, data=obs_data))
+
+        # does not require explicitly specifying `config`
+        self.t = MapKeyToFloat(
+            observations=self.observations,
+        )
+
+    def test_Init(self) -> None:
+        self.assertEqual(len(self.t._parameter_list), 1)
+
+        p = self.t._parameter_list[0]
+
+        self.assertEqual(p.name, MapKeyToFloat.DEFAULT_MAP_KEY)
+        self.assertEqual(p.parameter_type, ParameterType.FLOAT)
+        self.assertEqual(p.lower, 1.0)
+        self.assertEqual(p.upper, 5.0)
+        self.assertTrue(p.log_scale)
+
+        with self.subTest("infer parameter type"):
+            observations = []
+            for trial_index, width, height, steps in _enumerate():
+                obs_feat = ObservationFeatures(
+                    trial_index=trial_index,
+                    parameters={"width": width, "height": height},
+                    metadata={
+                        "foo": 42,
+                        "bar": int(steps),
+                    },
+                )
+                obs_data = ObservationData(
+                    metric_names=[], means=np.array([]), covariance=np.empty((0, 0))
+                )
+                observations.append(Observation(features=obs_feat, data=obs_data))
+
+        # test that one is able to override default config
+        with self.subTest(msg="override default config"):
+            t = MapKeyToFloat(
+                observations=self.observations,
+                config={
+                    "parameters": {MapKeyToFloat.DEFAULT_MAP_KEY: {"log_scale": False}}
+                },
+            )
+            self.assertDictEqual(t.parameters, {"steps": {"log_scale": False}})
+
+            self.assertEqual(len(t._parameter_list), 1)
+
+            p = t._parameter_list[0]
+
+            self.assertEqual(p.name, MapKeyToFloat.DEFAULT_MAP_KEY)
+            self.assertEqual(p.parameter_type, ParameterType.FLOAT)
+            self.assertEqual(p.lower, 1.0)
+            self.assertEqual(p.upper, 5.0)
+            self.assertFalse(p.log_scale)
+
+    def test_TransformSearchSpace(self) -> None:
+        ss2 = deepcopy(self.search_space)
+        ss2 = self.t.transform_search_space(ss2)
+
+        self.assertSetEqual(
+            set(ss2.parameters),
+            {"height", "width", MapKeyToFloat.DEFAULT_MAP_KEY},
+        )
+
+        p = assert_is_instance(
+            ss2.parameters[MapKeyToFloat.DEFAULT_MAP_KEY], RangeParameter
+        )
+
+        self.assertEqual(p.name, MapKeyToFloat.DEFAULT_MAP_KEY)
+        self.assertEqual(p.parameter_type, ParameterType.FLOAT)
+        self.assertEqual(p.lower, 1.0)
+        self.assertEqual(p.upper, 5.0)
+        self.assertTrue(p.log_scale)
+
+    def test_TransformObservationFeatures(self) -> None:
+        observation_features = [obs.features for obs in self.observations]
+        obs_ft2 = deepcopy(observation_features)
+        obs_ft2 = self.t.transform_observation_features(obs_ft2)
+
+        self.assertEqual(
+            obs_ft2,
+            [
+                ObservationFeatures(
+                    trial_index=trial_index,
+                    parameters={
+                        "width": width,
+                        "height": height,
+                        MapKeyToFloat.DEFAULT_MAP_KEY: steps,
+                    },
+                    metadata={"foo": 42},
+                )
+                for trial_index, width, height, steps in _enumerate()
+            ],
+        )
+        obs_ft2 = self.t.untransform_observation_features(obs_ft2)
+        self.assertEqual(obs_ft2, observation_features)
+
+    def test_TransformObservationFeaturesWithEmptyParameters(self) -> None:
+        obsf = ObservationFeatures(parameters={})
+        self.t.transform_observation_features([obsf])
+
+        p = self.t._parameter_list[0]
+        self.assertEqual(
+            obsf,
+            ObservationFeatures(parameters={MapKeyToFloat.DEFAULT_MAP_KEY: p.upper}),
+        )
diff --git a/sphinx/source/modelbridge.rst b/sphinx/source/modelbridge.rst
index b7edec63bce..c35831f380a 100644
--- a/sphinx/source/modelbridge.rst
+++ b/sphinx/source/modelbridge.rst
@@ -319,6 +319,15 @@ Transforms
     :undoc-members:
     :show-inheritance:
 
+
+`ax.modelbridge.transforms.map\_key\_to\_float`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. automodule:: ax.modelbridge.transforms.map_key_to_float
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
 `ax.modelbridge.transforms.rounding`
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~