Serena Jameka Williams (born September 26, 1981) is an American professional tennis player. The Women's Tennis Association (WTA) ranked her world No. 1 in singles on eight separate occasions between 2002 and 2017. She reached the No. 1 ranking for the first time on July 8, 2002. On her sixth occasion, she held the ranking for 186 consecutive weeks, tying the record set by Steffi Graf for the most consecutive weeks as No. 1 by a female tennis player. In total, she has been No. 1 for 319 weeks, which ranks third in the 'Open Era' among female players behind Steffi Graf and Martina Navratilova. Many commentators, players and sports writers regard her as the greatest female tennis player of all time.

Williams holds the most Grand Slam titles in singles, doubles, and mixed doubles combined amongst active players. Her 39 major titles puts her joint-third on the all-time list and second in the Open Era: 23 in singles, 14 in women's doubles, and two in mixed doubles. She is the most recent female player to have held all four Grand Slam singles titles simultaneously (2002–03 and 2014–15) and the third player to achieve this record twice after Rod Laver and Steffi Graf. She is also the most recent player to have won a Grand Slam title on each surface (hard, clay and grass) in one calendar year. She is also, together with her sister Venus, the most recent player to have held all four Grand Slam women's doubles titles simultaneously (2009–10).

Her total of 23 Grand Slam singles titles marks the record for the most Grand Slam wins by a tennis player in the Open Era, and is second on the all-time list behind Margaret Court (24). She is the only tennis player in history (man or woman) to have won singles titles at least six times in three of the four Grand Slam tournaments, and the only player ever to have won two Grand Slams seven times each (Wimbledon titles and Australian Open titles). She is also the only tennis player to have won ten Grand Slam singles titles in two separate decades. She has won an all-time record of 13 Grand Slam singles titles on hard court. Williams holds the Open Era record for most titles won at the Australian Open (7) and shares the Open Era record for most titles won at the US Open with Chris Evert (6). She also holds the all-time record for the most women's singles matches won at the Grand Slams with 319 matches.

Williams has won 14 Grand Slam doubles titles, all with her sister Venus, and the pair are unbeaten in Grand Slam doubles finals. As a team, she and Venus have the third most women's doubles Grand Slam titles, behind the 18 titles of Natasha Zvereva (14 with Gigi Fernández) and the record 20 titles won by Martina Navratilova and Pam Shriver. Williams is also a five-time winner of the WTA Tour Championships in the singles division. She has also won four Olympic gold medals, one in women's singles and three in women's doubles"an all-time record shared with her sister, Venus. The arrival of the Williams sisters has been credited with ushering in a new era of power and athleticism on the women's professional tennis tour. Earning almost $29 million in prize money and endorsements, Williams was the highest paid female athlete in 2016. She repeated this feat in 2017 when she was the only woman on Forbes' list of the 100 highest paid athletes with $27 million in prize money and endorsements. She has won the 'Laureus Sportswoman of the Year' award four times (2003, 2010, 2016, 2018), and in December 2015, she was named Sportsperson of the Year by Sports Illustrated magazine.