Mel B dances 'Gangnam Style' with Psy on 'X Factor' Australia - video
The public had the chance to vote for which judge they wanted to perform with the South Korean popstar.
Viewers chose the former Spice Girl, who was recently a guest judge on The X Factor UK, over Ronan Keating, Guy Sebastian and Natalie Bassingthwaighte.
"My daughter will kill me if I don't do this," Mel B said as she got up on stage.
She said after the performance: "My daughter said she'd cook me breakfast in bed on Friday. I have to say you are a legend. I do feel honoured. I thought my legs were about to snap!"
'Gangnam Style' is currently number one on the ARIA singles chart in Australia.