The 31-year-old extended her winning streak against the Russian to nine matches, holding her serve throughout and breaking the second seed three times.
Williams has already won Wimbledon, Olympic gold and US Open in 2012, and the American admitted she has enjoyed a great year after a slow start.
“I actually thought about the last time we played at the WTA Championships, and she won, and she’s also ranked ahead of me right now, so I really had nothing to lose,” Williams said.
“I feel great right now. This was exactly my dream and I’m so happy I was able to achieve it.
“And the Turkish fans here are so amazing, so nice and so supportive – I’ve never seen so many signs with my name on them. I can’t even explain how that makes me feel.”
She added: “I would say it’s pretty much the best season of my career, a little bit of a slow start though, so I have to give it to 2002 – but there’s always 2013.”
Sharapova was also content with her season despite losing the Istanbul showpiece after the 25-year-old was victorious at Roland Garros – her first Grand Slam title since recovering from a serious shoulder injury.
“I’ve got to keep moving forward,” said Sharapova. “I’m proud of my year. I’m proud of my consistency. I’m proud I’m moving in the right direction in terms of improving my game and where I see it.
“I’ve accomplished a lot of things this year I really wanted, and wanted to get back in my career.
“A lot of great memories. Some difficult ones, but a lot of really positive thoughts moving into next year.”
MORE: The latest football news
MORE: The latest tennis news
BIOGRAPHY: Marcus Rashford