| Mary Smith Brook | |
| Pushpin Map: | New York |
| Pushpin Map Caption: | Location of the mouth of Mary Smith Brook |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | State |
| Subdivision Name2: | New York |
| Subdivision Type4: | County |
| Subdivision Name4: | Delaware |
| Source1 Coordinates: | 42.0375°N -74.805°W |
| Mouth: | Beaver Kill |
| Mouth Coordinates: | 42.0056°N -74.7879°W[1] |
| Mouth Elevation: | 1578feet |
Mary Smith Brook is a river in Delaware County, New York. It flows into Beaver Kill northwest of Lewbeach.