Williams came within two matches of tennis’ first calendar-year Grand Slam since 1988, a bid that ended with a semi-final loss at the US Open.
In all, the 34-year-old American won 53 and lost three during 2015, clinching five titles, including the Australian Open, French Open and Wimbledon.
Williams was No 1 in the WTA rankings all season.
“She was the most deserving person for the award. She had an amazing year.
“The way she won her events; the fact that she’s done this for so many years at such a high level,” said Paul Fichtenbaum, editor of the Sports Illustrated Group.
Runner Mary Decker in 1983 was the last female athlete to earn the magazine’s award by herself.