USS Amaranthus was a screw steamer acquired by the Union Navy during the American Civil War.

Amaranthus a wooden-hulled screw tug built at Philadelphia, Pennsylvania, in 1864 by Bishop, Son, and Company—was purchased by the Navy there as Christiana on 1 July 1864.

On the night of 9 and 10 September, she sighted a steamer attempting to run out of Charleston and fired repeatedly at the blockade runner which, nevertheless, escaped to sea.

On both occasions, Confederate shore batteries at Fort Moultrie fired upon the Union blockaders; a spent 10-inch shell struck Amaranthus' starboard counter, damaging the tug sufficiently to require her to enter a nearby inlet for repairs.

This article incorporates text from the public domain Dictionary of American Naval Fighting Ships.