"America's Got Talent" has added a FOURTH JUDGE for its upcoming season: HEIDI KLUM.  She'll join returning judges HOWARD STERN and HOWIE MANDEL, along with MEL B from the SPICE GIRLS, who's replacing SHARON OSBOURNE as the token British judge that every reality competition show is required to have.

The show starts taping its audition rounds TODAY in New Orleans. (TMZ has a photo from the set, which includes Heidi's own personal "X".)