TMZ has learned Rachel Uchitel has signed a deal to get naked for Playboy Magazine -- but there's a full-frontal catch.



Rachel hedged when we asked her about a Playboy offer on TMZ Live on Friday -- but we found out that Tiger Woods' #1 mistress has already struck an agreement.  A source close to the mag tells us Rachel has the right to pull out any time before the shoot goes down -- which is in three weeks.


We're also told Rachel won't be baring it all -- just the backside ... and the topside.

No word on when the spread will run in the mag.