Martina Navratilova (Czech: Martina Navrátilová [ˈmarcɪna ˈnavraːcɪlovaː] (listen); née Šubertová [ˈʃubɛrtovaː]; born October 18, 1956) is a Czech–born American former professional tennis player. Widely considered among the greatest tennis players of all time, Navratilova won 18 major singles titles, 31 major women's doubles titles, and 10 major mixed doubles titles, for a combined total of 59 major titles, the most in the Open Era. Alongside Chris Evert, her greatest rival, Navratilova dominated women's tennis in the 1970s and 1980s.

Navratilova was ranked as the world No. 1 in singles for a total of 332 weeks (second only to Steffi Graf), and for a record 237 weeks in doubles, making her the only player in history to have held the top spot in both disciplines for over 200 weeks. She won 167 top-level singles titles and 177 doubles titles, both the Open Era records. She won a record six consecutive singles majors across 1983 and 1984 while simultaneously winning the Grand Slam in doubles. Navratilova claims the best professional season winning percentage, 98.8% in 1983 (going 98–1 for the season), and the longest all-surface winning streak of 74 straight match wins. She reached the Wimbledon singles final 12 times, including for nine consecutive years from 1982 through 1990, and won the title a record nine times. Navratilova is one of the three tennis players, along with Margaret Court and Doris Hart, to have accomplished a career Grand Slam in singles, doubles, and mixed doubles, called the career "Boxed Set". She won her last major title, the mixed doubles crown at the 2006 US Open, shortly before her 50th birthday, and 32 years after her first major title in 1974.

Originally from Czechoslovakia, Navratilova was stripped of her citizenship when, in 1975 at age 18, she asked the United States for political asylum and was granted temporary residence. She became a US citizen in 1981. On January 9, 2008, Navratilova acquired Czech citizenship, thus becoming a dual citizen. She stated she has not renounced her U.S. citizenship nor does she plan to do so, and that reclaiming Czech nationality was not politically motivated. Navratilova has been openly lesbian since 1981, and has been an activist for exclusively LGB issues, she is a prominent anti-trans activist, including supporting anti-trans legislation and being patron of an exclusively anti-trans organisation The Lesbian Project.