Iowa Highway 51 is a state highway that runs from north to south in northeast Iowa. Contained entirely by Allamakee County, it is only 11 miles (18 km) in length. Its southern terminus is at U.S. Highway 52 and U.S. Highway 18 in Postville. Its northern terminus is at Iowa Highway 9 near Waukon.

Iowa Highway 51 begins at an intersection with US 18 and US 52 in Postville. It leaves Postville heading in an almost directly northward alignment. It passes by a number of small streets along the way; there are no major intersections along the route. Southwest of Waukon, it terminates at an intersection with IA 9.

When it was originally designated, IA 9 was not a paved highway. The route was fully paved by 1935, with the segment from the Yellow River south to Postville being repaved in 1969. Its initial alignment also extended the highway into Waukon, but was shortened to end at an intersection with the old alignment of IA 13 (changed to IA 76 in 1969) on December 8, 1924. The highway's alignment again changed in December 1968, when it was truncated to run from Postville to an intersection with IA 9, which is the present-day alignment.

The entire route is in Allamakee County.