The funnyman joined comedian Jimmy Kimmel and country singer Gary Allan on late-night show Jimmy Kimmel Live! for verses of Tom Petty's Free Fallin' and Freedom by George Michael, which had been altered lyrically to urge German officials to release the pop star's pet back to his care.
Bieber was caught trying to smuggle the monkey into Germany without the correct paperwork last weekend (30-31Mar13).
Brand rapped, "This is an important issue about primate freedom," and added, "That monkey's really rare/It belongs in Justin's care."
The Brit also used his appearance on the show to campaign for empty Tv studios to be used to shelter the homeless.
Brand explained he had tried the idea in London with a homeless pal called Crazy Al, but it hadn't worked out because Al was "a bit of a loose cannon".
He explained, "He didn't observe all the rules and it was like a butcher's shop in there."
But he isn't giving up on the initiative and he is hoping studio bosses in America will support his plan: "I will continue with my campaign to allow homeless people to occupy Tv studios at night."