A longtime assistant to Jasper Johns pleaded guilty on Wednesday to selling 22 artworks he stole from the artist's Connecticut studio.

The plea deal followed a separate case, earlier this year, in which another former collaborator of Mr. Johns pleaded guilty to selling works unauthorized by the artist. The cases highlight the risks and rewards of forging works by living artists as contemporary art prices soar. (The record price for a work by Mr. Johns, set at Christie's in 2010, is $28.6 million.)