| Official Name: | Lake St. Clair, Missouri |
| Settlement Type: | Census-designated place |
| Pushpin Map: | Missouri#USA |
| Pushpin Label: | Lake St. Clair |
| Coordinates: | 38.3225°N -90.9986°W |
| Subdivision Type: | Country |
| Subdivision Name: | United States |
| Subdivision Type1: | State |
| Subdivision Name1: | Missouri |
| Subdivision Type2: | County |
| Subdivision Name2: | Franklin |
| Subdivision Type3: | Township |
| Subdivision Name3: | Central |
| Area Footnotes: | [1] |
| Area Total Sq Mi: | 0.84 |
| Area Land Sq Mi: | 0.72 |
| Area Water Sq Mi: | 0.12 |
| Area Total Km2: | 2.17 |
| Area Land Km2: | 1.87 |
| Area Water Km2: | 0.30 |
| Population As Of: | 2020 |
| Population Density Km2: | 440.10 |
| Timezone: | Central (CST) |
| Utc Offset: | -6 |
| Timezone Dst: | CDT |
| Utc Offset Dst: | -5 |
| Elevation Ft: | 705 |
| Postal Code Type: | ZIP Code |
| Postal Code: | 63077 (Saint Clair) |
| Area Code: | 636 |
| Blank Name: | FIPS code |
| Blank Info: | 29-40039 |
| Blank1 Name: | GNIS feature ID |
| Blank1 Info: | 2806400 |
| Population Total: | 825 |
| Unit Pref: | Imperial |
| Population Density Sq Mi: | 1139.50 |
Lake St. Clair or Lake Saint Clair is an unincorporated community and census-designated place (CDP) in Franklin County, Missouri, United States. It is in the southeast part of the county, surrounding a lake impounded on a tributary of Hoosier Creek, a south-flowing tributary of the Meramec River. The community is bordered to the north by the city of Saint Clair and is southwest of St. Louis.
Lake St. Clair was first listed as a CDP prior to the 2020 census.