The forward admitted attacking John Friel in Glasgow's Queen Street on November 3, 2010. The 23-year-old Scotland international punched him on the head and body during the attack.
He admitted the assault when he appeared at Glasgow Sheriff Court on Monday, a court official said. Sentence was deferred until April 30 for reports.
Goodwillie, from Stirling, moved from Dundee United to Blackburn last summer for a fee of around £2 million.