| Official Name: | St. Clair, Pennsylvania |
| Settlement Type: | Census-designated place |
| Pushpin Map: | Pennsylvania#USA |
| Pushpin Label: | St. Clair |
| Pushpin Label Position: | top |
| Coordinates: | 40.4783°N -78.2283°W |
| Subdivision Type: | Country |
| Subdivision Name: | United States |
| Subdivision Type1: | State |
| Subdivision Name1: | Pennsylvania |
| Subdivision Type2: | County |
| Subdivision Name2: | Blair |
| Subdivision Type3: | Township |
| Subdivision Name3: | Catharine |
| Unit Pref: | Imperial |
| Area Footnotes: | [1] |
| Area Total Km2: | 0.08 |
| Area Land Km2: | 0.08 |
| Area Water Km2: | 0.0 |
| Population As Of: | 2020 |
| Timezone: | Eastern (EST) |
| Utc Offset: | -5 |
| Timezone Dst: | EDT |
| Utc Offset Dst: | -4 |
| Elevation Ft: | 876 |
| Postal Code Type: | ZIP Code |
| Postal Code: | 16693 (Williamsburg) |
| Area Code: | 814/582 |
| Blank Name: | FIPS code |
| Blank Info: | 42-67220 |
| Blank1 Name: | GNIS feature ID |
| Blank1 Info: | 2805565 |
St. Clair is an unincorporated community and census-designated place (CDP) in Blair County, Pennsylvania, United States. It was first listed as a CDP prior to the 2020 census.
The CDP is in eastern Blair County, in the southern part of Catharine Township. It is bordered to the southwest by the Frankstown Branch of the Juniata River, which forms the border with Woodbury Township to the south. Pennsylvania Route 866 passes through St. Clair, leading southeast 2miles to Williamsburg and northwest to U.S. Route 22 near Point View.
St. Clair sits at the southern base of Short Mountain, which rises above the community to an elevation of .