The American was broken in the second set but wrapped up the victory after the Serbian eighth seed had to take a medical time out for a hip problem.
“I’m really excited – I didn’t think I’d win this tournament starting out, so I’m really happy to be here holding the trophy,” said Williams, who having also won Beijing in 2004, becomes the event’s second two-time champion after Svetlana Kuznetsova.
“Congratulations to Jelena for such a great tournament as well.
“I really love tennis so much and it’s fans like the ones here in China who are so exciting to come to. The fans make me want to play, and they’re so great here, so coming here is such a great opportunity.”
World No1 Williams becomes the first player to net 10 WTA titles in a season since Justine Henin won 10 in 2007.
It was the US Open champion’s 73rd match win of the year, giving her the most victories in a season of any player since 2003.
Jankovic, who is set to return to the top 10 at No8 in the new rankings – her highest position since May 2011, conceded that Williams was a level above her.
“I had a great week playing in this beautiful venue,” said Jankovic.
“I always enjoy coming back here to the China Open – it’s a pleasure for me to play in front of you all in the finals again. Serena was just too good today, and big congratulations to her.”
MORE: The latest football news
MORE: The latest tennis news