The athlete competed in the 4x200-metre freestyle relay with his fellow Team U.S.A. stars Ryan Lochte, Conor Dwyer and Ricky Berens and swam his way into the record books as they clinched victory, beating runners-up France by more than three seconds. China came in third place to take bronze.
The latest gold medal makes Phelps the first Olympic contestant to have 19 medals to his name - 15 of which were awarded for first place - snatching the title from former Soviet gymnast Larisa Latynina, who won 18 honours.
Earlier on Tuesday, Phelps added to his haul by landing silver in the 4x100-metre freestyle relay. And the 27 year old has two more chances to boost his lead as an all-time record holder - he will also compete in the forthcoming 100-metre butterfly and 200-metre individual medley finals.
Phelps has previously insisted the London 2012 Games will be his last.