You can tell Nelly’s about to release a CD — all of a sudden he’s appearing in unexpected places as “Brass Knuckles” inches towards its Sept. 16 release date.

“Nelly Climbs Into Diddy’s Underpants” is the headline-of-the-week E! News used in reporting the St. Louis rapper is the newest spokesperson for Diddy’s Sean John Underwear.

He’ll appear in a Randee St. Nicholas -photographed print campaign launching in the fall.

In a statement, Diddy said: “To me, Nelly is the quintessential definition of the Sean John man. And let’s be honest, all of the ladies out there are going to be thanking us for this campaign.”

Diddy’s referring to the fact Nelly’s revamped bod is killer, and he’s not shy about showing it off.

Also, Nelly is set to appear in several episodes of “CSI: NY” this fall, in which he’ll play a club owner/informant. A representative for Nelly says his TV gig will be ushered in with a viewing party at the Skybox on Sept. 24, with details coming.