Williams, 32, went 78-4 in 2013 with 11 tournament titles. She won the French Open and U.S. Open in addition to the WTA Championships. That success allowed her to set a record with more than $12.4 million won in 2013.
She previously won the honor in 2002, '08, '09 and '12. Since the award was initiated in 1977 only Steffi Graff (eight times) and Martina Navratilova (seven) have won it more than Williams.
The No. 1-ranked team of Sara Errani and Roberta Vinci were selected the WTA's Doubles Team of the Year. They won the Australian Open women's doubles title.
Simona Halep was selected Most Improved, Eugenia Bouchard was selected Best Newcomer and Alisa Kleybanova was picked Comeback Player of the Year for a second consecutive season. She was diagnosed with Hodgkin's disease two years ago.