The pair laid into each other and had to be pulled apart by shocked team-mates in another embarrassing incident for the club, which was witnessed by spectators and photographers.
Incredibly, the two are the best of friends.
The row was sparked by a pass from England defender Richards, which did not reach Balotelli.
The Italian striker said: "If you won't pass to my feet, I won't move."
Richards, who had been unimpressed by Balotelli's contribution to the training session, replied: "Grow up Mario."
When Balotelli burst into a string of expletives in Italian, Richards launched himself at his friend.
They were then brought together by manager Roberto Mancini for a stern lecture in front of team-mates, and immediately made up, by shaking hands and laughing off the incident.
Richards played down the row in Twitter comments afterwards, when he said:
"Me & Mario are all good! these things happen in training & we shook hands after. it shows passion! #ctid"