MAROON 5 singer ADAM LEVINE and Victoria's Secret model BEHATI PRINSLOO were married Saturday night in Cabo San Lucas, Mexico . . . by JONAH HILL.


Adam and Jonah are longtime friends, and a source says, quote, "Jonah was hysterical. He was telling so many jokes, but then in the middle would be so sweet and sincere."


Guests included Robert Downey Jr.Jason Segel and Victoria's Secret model Erin Heatherton . . . plus Maroon 5 members James Valentine and Jesse Carmichael.  Some of the bridesmaids were Victoria's Secret models.


The reception featured several musical performances, including one by STEVIE NICKS, who's mentoring Adam's team on the next season of "The Voice".


They're honeymooning in South Africa.


(As a side note, Jonah Hill was also responsible for Adam getting the role of Keira Knightley's cheating boyfriend in the movie "Begin Again".  You can listen to the director talk about that here.)