The USS Eilonwy was a Federation starship in service during the latter half of the 23rd century. In 2275, she was docked at Starbase 18 as a possible candidate to test the new inversion drive technology. She lost out to the USS Enterprise. (TOS novel: The Wounded Sky)

It is unclear to what class the Eilonwy belonged, but from the context, it is likely that she was a Constitution-class vessel.