Michael Coombs, also known as rapper Mike GLC, pleaded guilty to supplying the Class A drug when he appeared at London's Southwark Crown Court alongside the former N-Dubz star.
Contostavlos, 26, is due to stand trial after she denied being concerned in brokering the alleged drug deal which was exposed by the Sun on Sunday.
It is claimed the singer told undercover reporter Mazher Mahmood, dubbed the "fake sheikh", that she could arrange for him to buy cocaine from Coombs between May 20 and May 24 last year.
Contostavlos, of Friern Barnet, north London, denies helping Mr Mahmood to obtain 0.5oz (13.9g) of the Class A drug for £860.
Wearing a white short sleeved shirt and black trousers, Coombs, 36, of Enfield, north London, pleaded guilty to a charge of supplying the drug on May 23 last year at the five star Dorchester Hotel in London's exclusive Mayfair district.
Contostavlos appeared in the dock wearing a white blazer, black dress with a colourful floral pattern, and with her black hair worn down in loose curls.
She sat next to Coombs in the glass fronted dock and spoke clearly to confirm her name and plead not guilty.
Bailing Coombs, Judge Alistair McCreath told the rapper he would be sentenced at the end of Contostavlos' trial.
He said: "Michael Coombs, I'll sentence you at the conclusion of the proceedings against your co-defendant, whatever that conclusion may be."
The prosecution were due to open their case today.