Flatbush includes the southernmost portion of Prospect Park.

The neighborhoods of Flatbush extend south from the old Brooklyn City Line north of the southern edges of Prospect Park, the Brooklyn Botanic Garden and Empire Boulevard. The southern border of Flatbush neighborhoods is approximately on the line of the Long Island Rail Road's Bay Ridge Line, which runs to the south of Avenue H, the campus of Brooklyn College, and "The Junction" where Flatbush and Nostrand Avenues intersect. "The Junction" is also the location of Brooklyn College–Flatbush Avenue station, the southern terminal of the IRT Nostrand Avenue Line (2 5) of the New York City Subway and the Junction Mall.^[6] Flatbush's eastern border is roughly around New York Avenue , while its western border is Coney Island Avenue.

Neighborhoods within Flatbush include the planned communities of Prospect Park South, Beverley Square West, Beverley Square East, Prospect-Lefferts Gardens, Ditmas Park, Fiske Terrace and Albemarle-Kenmare Terrace. Bordering Flatbush on the north are the community of Crown Heights and the former neighborhood of Pigtown. On the east, within the old town of Flatbush, is East Flatbush, on the west are Kensington and Parkville (formerly Greenfield), and on the south is Midwood. Many consider Midwood to be a part of Flatbush, but historically it was part of the neighboring former towns of New Utrecht, Gravesend and Flatlands.