The pair, who spent the 2006-07 season together at the Etihad Stadium, got into an angry back-and-forth after Barton tweeted a rumor that Marseille teammate Loic Remy was set to choose a move to QPR instead of Newcastle.
Hamann took exception to this, and the pair spent time trading insults on the social networking website, before Barton took to his account this morning to apologize.
He tweeted: “I've slept on it. I have no right to question a man I hardly know. I must, therefore apologize for my part in last night's heated exchange.”
The pair mentioned details about their time as teammates and were engaged in the row for just over an hour.