The victory was the 26-year-old's ninth Grand Slam title, and her third triumph at the U.S. Open.
Although the match was the first Grand Slam final for Jankovic, 23, the Serb shook off any nerves to start well, but Williams hit back to take the opening set. Williams then saved four set points in the second set before going on to claim a straight-sets victory.
"I'm so excited," Williams said after the victory. "I wasn't even going for No. 1, and it's just like an added bonus."
The final was due to be played on Saturday, but was postponed due to the tail end of Tropical Storm Hanna.
"I was proud to be in the finals, but I'm always disappointed when I lose," said Jankovic, admitting that "Serena was a better player tonight."
