Justin Bieber -- THE ROAST IS ON ... Bring On Jeff Ross!

8:30 AM PT -- The roast was Bieber's idea! We've learned Justin actually called Comedy Central to say he wanted the roast to be a celebration for his 21st birthday ... which is March 1.

Like a tattooed lamb to the slaughter, Justin Bieber has agreed to be the guest of honor at a Comedy Central Roast in March ... and you can already hear Jeff Ross licking his chops.

Ryan Seacrest broke the news on Twitter ... saying, "Yes I got confirmation @justinbieber is getting roasted by @ComedyCentral in march...love u justin but it's time lol #BieberRoast."

Word on the street is that Bieber was game to be roasted for a while ... and had been hoping to work out a deal with the network.

Now, it seems like everything's a go ... and it'll be interesting to see which comics come out to burn Bieber.

As for roast topics, it's not like there's a shortage of material, for example:

-- Pissing in mop buckets
-- Egging houses
-- Lambo arrest
-- Fighting Orlando Bloom
-- Dropping the N-word
-- Calvin Klein modeling shots
-- Anne Frank shout out
-- Push-ups in jail
-- Fighting with Keyshawn Johnson
-- Fear of shirts
... and oh yeah, his music career.

Careful what you wish for.