Mats Arne Olof Wilander (Swedish: [ˈmats vɪˈlǎnːdɛr]; born 22 August 1964) is a Swedish former professional tennis player. He was ranked as the world No. 1 in men's singles by the Association of Tennis Professionals (ATP) for 20 weeks, including as the year-end No. 1 in 1988. Wilander won 33 career singles titles, including seven majors (three each at the French Open and Australian Open, and one at the US Open), and seven career doubles titles, including a major in men's doubles at Wimbledon.

Wilander's breakthrough came suddenly and unexpectedly when he won the 1982 French Open at the age of 17. Wilander won his fourth major singles title at the age of 20, the youngest man in history to have achieved the feat. In 1988, he won three of the four singles majors to finish the year ranked as the world No. 1. Wilander also won eight Grand Prix Super Series titles (1983–88), the precursors to the current ATP Masters 1000 tournaments. He was also a driving force behind Sweden's run of seven consecutive Davis Cup finals from 1983 to 1989, and three titles in 1984, 1985, and 1987. Wilander is one of seven men to have won major singles titles on grass courts, hard courts, and clay courts since the feat became achievable in 1978 (when the US Open was first played on hard courts). Wilander, Nadal, Djokovic and Alcaraz are the only men to have won at least two major singles titles on each of the three surfaces. Wilander retired from the sport in 1996.

In 1983, Wilander won the Jerring Award. In 2002, he was inducted into the International Tennis Hall of Fame.

Born in Växjö, Sweden, Wilander first came to the tennis world's attention when he won the French Open junior title, the European under-16 and under-18 championships, and the Orange Bowl under-16 event in Miami.

Wilander made his debut on the professional tour at the clay court tournament in Båstad, Sweden in 1980. In June 1981 he made his Grand Slam singles debut at Wimbledon, losing in the third round to John Fitzgerald. The Canberra Times said "16-year-old Mats Wilander, who many say will become the new Borg." In September 1981, he lost his only career match against Björn Borg, losing in the first round of the Geneva Open in straight sets. Wilander reached his first ATP final in November 1981 in Bangkok, losing in straight sets to Bill Scanlon.

Wilander surprised the tennis world at the 1982 French Open. As an unseeded player, he upset second seed Ivan Lendl in the fourth round, fifth seed Vitas Gerulaitis in the quarterfinals, fourth seed José Luis Clerc in the semifinals, and third seed Guillermo Vilas in a four-set final that lasted 4 hours and 47 minutes (the longest French singles final played up until that point) and was notable for its long rallies, the longest point taking 90 strokes. At the end of the semifinal against Clerc he requested replay of the match ball as he did not want to win the game due to a questionable referee decision. This was seen as an extraordinary display of fair play and garnered him the Pierre de Coubertin World Fair Play Trophy. He was the youngest ever male Grand Slam singles champion at 17 years, 9 months, a record since broken by Boris Becker and Michael Chang. In only his third entry in a Grand Slam tournament, Wilander also became the player who needed the fewest attempts to win one, a record since equaled by Gustavo Kuerten at the 1997 French Open. Wilander then lost in the fourth round at both Wimbledon, to Brian Teacher, and the US Open to Lendl. Wilander won three additional tournaments in 1982 and finished the year ranked no. 7. During that year, Wilander was awarded the Svenska Dagbladet Gold Medal.

Wilander returned to the French Open in 1983, where he lost to Yannick Noah in the final, after defeating John McEnroe in a quarterfinal. He lost in the third round at Wimbledon to Roscoe Tanner and in the quarterfinals of the US Open to Lendl. Wilander won his second Grand Slam title later that year at the Australian Open, played on grass at Kooyong Stadium, where he defeated McEnroe in a semifinal and Lendl in straight sets in the final, which was a 'basline battle". He won eight other tournaments in 1983, including two Grand Prix Championship Series titles, and finished the year ranked no. 4.

Wilander retained his Australian Open title in 1984, beating Stefan Edberg in the quarterfinals and Kevin Curren in the final. "I don't think I've ever played anyone on grass who consistently makes as many returns as he does from below the net. I felt the whole time I was volleying off my shoes or hitting a half volley and unless you hit a great half volley he goes for the passing shots" said Curren afterwards. He lost in the semifinals of the French Open to Lendl, the second round at Wimbledon to Pat Cash, and the quarterfinals of the US Open to Cash. He won three tournaments in 1984 including his third Championships Series title and again finished the year ranked no. 4.