The Patuxent River is a tributary of the Chesapeake Bay in the state of Maryland. Its source is located in the hills of the Maryland Piedmont near the intersection of four counties - Howard, Frederick, Montgomery and Carroll. Flowing in a generally southward direction, the Patuxent crosses the heavily urbanized corridor between Baltimore and Washington, D.C. and opens up into a navigable estuary near the town of Deep Landing in Calvert County, Maryland, finding the Chesapeake Bay about thirty miles later.

The river marks the boundary between Montgomery, Prince George's, Charles and St. Mary's Counties on the west and Howard, Anne Arundel, and Calvert Counties on the east.