| Official Name: | Mill Plain, Connecticut |
| Settlement Type: | Census-designated place |
| Pushpin Map: | Connecticut#USA |
| Pushpin Label: | Mill Plain |
| Coordinates: | 41.15°N -73.2692°W |
| Subdivision Type: | Country |
| Subdivision Type1: | State |
| Subdivision Name1: | Connecticut |
| Subdivision Type2: | County |
| Subdivision Name2: | Fairfield |
| Subdivision Type3: | Town |
| Subdivision Name3: | Fairfield |
| Area Footnotes: | [1] |
| Area Total Km2: | 1.31 |
| Area Land Km2: | 1.26 |
| Area Water Km2: | 0.05 |
| Population As Of: | 2020 |
| Timezone: | Eastern (EST) |
| Utc Offset: | -5 |
| Timezone Dst: | EDT |
| Utc Offset Dst: | -4 |
| Elevation Ft: | 25 |
| Postal Code Type: | ZIP Code |
| Postal Code: | 06824 (Fairfield) |
| Area Code: | 203/475 |
| Blank Name: | FIPS code |
| Blank Info: | 09-47990 |
| Blank1 Name: | GNIS feature ID |
| Blank1 Info: | 2805956 |
Mill Plain is a census-designated place (CDP) in the town of Fairfield, Fairfield County, Connecticut, United States. It is in the southern part of the town, directly north of the downtown area of Fairfield and northeast of Southport. It is bordered to the west by the Mill River, to the south by Interstate 95, to the east by Mill Plain Road, and to the north by Duck Farm Road.
Mill Plain was first listed as a CDP prior to the 2020 census.