South Lake Union is a neighborhood in Seattle, Washington, so named because it is at the south tip of Lake Union. It is bounded by Denny Way on the south, beyond which is Downtown, by Interstate 5 on the east, beyond which is Capitol Hill, by Aurora Avenue N. (Washington State Route 99) on the west, beyond which is Lower Queen Anne, and Valley Street, Lake Union, and E. Garfield Street on the north, beyond which are Westlake and Eastlake.

Its main thoroughfares are Valley, Mercer, and Broad Streets (east- and westbound) and Dexter, 9th, Westlake, and Fairview Avenues N. and Eastlake Avenue E. (north- and southbound).

South Lake Union east of Fairview Avenue N. is also known as Cascade.

Of note are the Fred Hutchinson Cancer Research Center and Shurgard Storage Centers, both headquartered in South Lake Union, the flagship store of R.E.I., located at Eastlake Avenue E. and John Street in the neighborhood's southeast corner, and Denny Park, the oldest park in the city.