All of the undivided, two-laned route is located in western Hancock County.

The road goes through a small forest, before turning northwest and entering Pearlington.

MS 604 goes through the center of Pearlington, and turns north at Levee Street.

[1] The route is part of the Westonia Senic Byway to Space, which starts at MS 607, travels on MS 604, and ends at Stennis Space Center's buffer zone near Pearlington.

[6] In 1936, a new, paved road was built from US 90 to south of Slidell, bypassing the town.