Mayweather (46-0, 26 KOs) took two cards 117-111 and 116-112, with the other judge posting a score of 114-114 as he improved to 16-0 in fights that have gone to 12 rounds.
Maidana (35-4, 31 KOs) dominated the early rounds as he had Mayweather on the ropes much of the time, but it wasn't enough in his first defense of his WBA title.
The first several rounds featured Maidana throwing a lot of punches and keeping Mayweather along the ropes. He caught the undefeated champ with some overhand clubs and a few rights, with Mayweather occasionally countering with some rights and body shots.
In the fourth round, an accidental headbutt opened a cut over Mayweather's right eye, the first time he had ever been cut in a fight, and it seemed to bother him for the rest of the round.
His corner team, though, was able to close the cut up and keep it from hampering him.
Mayweather started to get going in the sixth round, as he was able to connect on a hard right, and a big inside right uppercut in the round, and followed that with a few more hard rights in the seventh.
Maidana, who was talked to in a timeout by the ref during the third round for illegal shots, was again warned by the ref in the eighth round for another low blow.
The ninth round saw Mayweather continue to keep Maidana at a distance, and was able to connect on a right cross and a quick combination to the body.
In the 10th, Mayweather kept pushing Maidana back and hit the Argentinean with a big right to the head just before the bell.
The last two rounds saw the boxers continue to spar without letting the big blows land before the final bell sounded.
This was the third fight for Mayweather out of his 30-month, six-fight deal worth $200 million-plus that Mayweather signed with Showtime and its parent network CBS last February.