Lleyton Glynn Hewitt (born 24 February 1981) is an Australian former professional tennis player. He was ranked as the world No. 1 in men's singles by the Association of Tennis Professionals (ATP) for 80 weeks, including as the year-end No. 1 in 2001 and 2002. Hewitt won 30 singles titles and 3 doubles titles on the ATP Tour, including two singles majors at the 2001 US Open and 2002 Wimbledon Championships, a doubles major at the 2000 US Open, the 2001 and 2002 Tennis Masters Cups in singles, and led Australia to Davis Cup crowns in 1999 and 2003. In November 2001, Hewitt became (at the time) the youngest man to reach No. 1 in the ATP singles rankings, at the age of 20 years, 8 months and 26 days. He was also the runner-up at the 2004 US Open and 2005 Australian Open. He remains the most recent Australian man to win a singles major.

Hewitt was born in Adelaide, South Australia. His father, Glynn, is a former Australian rules football player, and his mother, Cherilyn, was a physical education teacher. He has a younger sister, Jaslyn, a former tennis coach and bodybuilder, and his brother-in-law (Jaslyn's husband) is Rob Shehadie. Hewitt also played Australian Football until the age of 13, when he decided to pursue a tennis career. His junior tennis club was Seaside Tennis Club in Henley Beach. He was also coached by Peter Smith at Denman Tennis Club in Mitcham.

As a junior Hewitt posted a 44–19 record in singles and reached as high as No. 17 in the world in 1997 (and No. 13 in doubles).

Hewitt commenced his professional career in 1998. He became one of the youngest winners of an Association of Tennis Professionals (ATP) tournament when he won the 1998 Next Generation Adelaide International, defeating Jason Stoltenberg in the final, having defeated Andre Agassi in the semi-finals. Both Aaron Krickstein winning Tel Aviv in 1983 and Michael Chang winning San Francisco in 1988 were younger than Hewitt when they claimed their first ATP title. Hewitt then left Immanuel College to concentrate on his tennis career. He was an Australian Institute of Sport scholarship holder.

In 2000, Hewitt reached his first Grand Slam final at the Wimbledon mixed doubles partnering Belgian Kim Clijsters, his then girlfriend. They lost the match, to Americans Kimberly Po and Donald Johnson. Hewitt later won his first Grand Slam title at the US Open when he along with Max Mirnyi claimed the men's doubles championship, thus becoming the youngest male (at 19 years, 6 months) to win a Grand Slam doubles crown in the open era. At the end of the year, Hewitt became the first teenager in ATP history to qualify for the year-end Tennis Masters Cup (ATP World Tour Finals).

Hewitt started off the 2001 season well by winning the title in Sydney. He went on to win back-to-back tournaments in Queen's and 's-Hertogenbosch. He captured his first Grand Slam singles title at the US Open, comprehensively defeating former world No. 1 Yevgeny Kafelnikov in the semi-finals, and four-time champion Pete Sampras the next day in straight sets (7-6, 6-1, 6-1). This win made Hewitt the most recent male player to win a Grand Slam singles and doubles title during his career. The Australian went on to win the Tokyo Open, and he again qualified for the year-end Tennis Masters Cup, held in Sydney. During the tournament, Hewitt won all matches in his group; with his round-robin victory against Pat Rafter, Hewitt secured the year-end world No. 1 ranking for the first time. Hewitt went on to defeat Sébastien Grosjean in the final to take the title and cement his position as the world number one.

2002 started with disappointment for Hewitt, as he lost in the first round of the Australian Open, his home Grand Slam. This was the first time the number 1 seed had lost in the opening round of the Australia Open, although Hewitt was still recovering from chickenpox. The next few months proved more successful for Hewitt and he cemented his position as the best player in the world by winning three titles: San Jose, Indian Wells and Queen's. His victory against Andre Agassi in the final of San Jose was heralded as one of the greatest matches of the season. He followed his 2001 US Open win by capturing the Wimbledon singles title. He defeated Jonas Björkman, Grégory Carraz, Julian Knowle, Mikhail Youzhny, Sjeng Schalken and dispatched home favourite Tim Henman - a rematch of their recent Queen's final - before dominating first-time finalist David Nalbandian in straight sets in the championship match; Hewitt lost only two sets (both to Schalken) throughout the championship. His victory reinforced the idea that, although the tournament had tended to be dominated by serve-and-volleyers, a baseliner could still triumph on grass (Hewitt was the first 'baseliner' to win the tournament since Agassi in 1992). Hewitt was the last man outside the so-called "Big Four" to win Wimbledon until Carlos Alcaraz in 2023.

For his third straight year, he qualified for the year-end Tennis Masters Cup, held in Shanghai, and successfully defended his title by defeating Juan Carlos Ferrero in the final. Hewitt's win helped him finish the year ranked No. 1 for a second straight year, becoming the seventh man to achieve this feat.