Xylophanes fassli is a moth of the family Sphingidae. It is known from Bolivia. It is similar to Xylophanes rothschildi, but the discal spot on the forewing upperside is larger, the deeper olive-green shading at the base is absent and the darker green oval patch distal to the discal spot is smaller. The pink coloration of the fringe of the inner margin from the base to the postmedian line is visible only on close examination or with the use of a magnifying glass. The larvae probably feed on Rubiaceae and Malvaceae species.