Serena Jameka Williams (born September 26, 1981) is an American inactive professional tennis player. Considered among the greatest tennis players of all time, she was ranked world No. 1 in singles by the Women's Tennis Association (WTA) for 319 weeks, including a joint-record 186 consecutive weeks, and finished as the year-end No. 1 five times. She won 23 Grand Slam singles titles, the most by any player in the Open Era, and the second-most of all time. She is the only player, male or female, to accomplish a Career Golden Slam in both singles and doubles.

Along with her older sister Venus, Serena Williams was coached by her parents Oracene Price and Richard Williams. Turning professional in 1995, she won her first major singles title at the 1999 US Open. From the 2002 French Open to the 2003 Australian Open, she was dominant, winning all four major singles titles (each time over Venus in the final) to achieve a non-calendar year Grand Slam and the career Grand Slam, known as the 'Serena Slam'. The next few years saw her claim two more singles majors, but suffer from injury and decline in form. Beginning in 2007, however, she gradually returned to form despite continued injuries, retaking the world No. 1 singles ranking. Beginning at the 2012 Wimbledon Championships, Williams returned to dominance, claiming Olympic gold (completing the Career Golden Slam in singles) and winning eight out of thirteen singles majors, including all four in a row from 2014–15 to achieve a second "Serena Slam". At the 2017 Australian Open, she won her 23rd major singles title, surpassing Steffi Graf's Open Era record. She then took a break from professional tennis after becoming pregnant and reached four major finals upon returning to play. In August 2022, Williams announced her impending "evolution" away from professional tennis and played what was expected to be her final match at the 2022 US Open.

Williams also won 14 major women's doubles titles, all with her sister Venus, and the pair was unbeaten in major doubles finals (the best unbeaten record in major finals in any discipline of the sport). The pair achieved a non-calendar year Grand Slam between the 2009 Wimbledon Championships and the 2010 French Open, which granted the sisters the doubles world No. 1 ranking. Serena won four Olympic gold medals, three in women's doubles—an all-time joint record in tennis, shared with her sister. The duo are the only women in the Open Era to win Olympic gold in both singles and doubles. She also won two major mixed doubles titles, both in 1998. She is the only singles player, male or female, to complete three Career Golden Slams – one in women's singles and two in same-sex doubles.

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. Serena holds a combined 39 major titles: 23 in singles, 14 in women's doubles, and two in mixed doubles. She is joint-third on the all-time list and second in the Open Era for total major titles. She is the most recent woman to simultaneously hold all four major singles titles (2002–03 and 2014–15), and the most recent woman to win the Surface Slam (major titles on hard, clay and grass courts in the same calendar year), doing so in 2015. She is also, with Venus, the most recent player to have simultaneously held all four major women's doubles titles (2009–10).

Williams was the world's highest paid woman athlete in 2016, earning almost $29 million. 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 won the Laureus Sportswoman of the Year award a record four times (2003, 2010, 2016, 2018), and in December 2015 was named Sportsperson of the Year by Sports Illustrated magazine. She is the highest-earning woman athlete of all time.