Gastón Norberto Gaudio (Spanish pronunciation: [ɡasˈtoŋ ˈɡawðjo]; born 9 December 1978) is an Argentine former professional tennis player. He won eight singles titles and achieved a career-high ATP singles ranking of world No. 5 in April 2005. Gaudio's most significant championship came at the 2004 French Open, when he defeated fellow Argentine Guillermo Coria from two sets down in the final.

Gaudio learned the game at the Temperley Lawn Tennis Club, and his first coach was Roberto Carruthers. He was the youngest of 3 children in his family. In addition to tennis Gaudio played football and rugby as a child and chose tennis to help out his parents financially when their business ran into economic problems.

Gaudio started playing tennis at the age of six. He finished as No. 2 in Argentine juniors in 1996 and turned professional the same year.

Gaudio finished as No. 2 junior in Argentina in 1996. Gaudio was ranked at 639 in the world in 1997.

In 1998 he reached four ATP Challenger finals during the second half of the year and won three of them. He won in Elche with a victory over fellow Argentine Diego Hipperdinger in July. He lost in Belo Horizonte to Brazilian Francisco Costa, and won in Santa Cruz with a victory over Ecuadorian Luis Morejón, both in August. He finished the year by winning in Santiago defeating Karim Alami and ranked world No. 138.

Gaudio won two consecutive Challengers in Nice and Espinho defeating Jacobo Díaz and Markus Hipfl, respectively. Gaudio's first notable performance was when he reached the third round at the French Open as a qualifier, so he won five matches total at the event, including coming back from two sets to love down in the second round against Bernd Karbacher to win, then losing to world No. 6 Àlex Corretja. He finished the year ranked No. 73.

2000 saw Gaudio establish himself on the main tour and win his only Challenger of the year in Braunschweig over countryman Franco Squillari. In addition to his Challenger title, Gaudio made the semifinals in Auckland, Santiago and, in his most impressive performance of the season, the Monte Carlo Masters, where he defeated Marat Safin, Félix Mantilla, Julien Boutter, and Juan Carlos Ferrero without dropping a set, before losing to Slovakia's Dominik Hrbatý in a tough three-set match. Gaudio also made the final of Stuttgart, again playing against fellow-Argentine Franco Squillari. Gaudio lost the final in five sets despite having beaten his opponent soundly in the Gstaad quarterfinals and in the Braunschweig finals earlier in the year (both on clay) and leading Squillari 2 sets to 1 in Stuttgart. Gaudio also represented Argentina in his first Olympic Games, losing to Vladimir Voltchkov of Belarus in the first round. He finished the year ranked No. 34.

Gaudio started his 2001 in poor fashion, losing his first four matches of the season to Vladimir Voltchkov, former French Open finalist Andrei Medvedev, three-time French Open winner Gustavo Kuerten, and, in his Davis Cup debut, Mexican Bruno Echagaray. Gaudio soon went back to his winning ways, however, reaching the final of Viña del Mar, losing to bitter rival and countryman Guillermo Coria. Gaudio would avenge that defeat to Coria in a hard-fought victory in the quarterfinals of Buenos Aires, which involved both players making rude gestures and insulting each other regularly. After this victory, Gaudio lost in the semifinals to José Acasuso. In the American hard-court swing after the 2001 Australian Open, he made the quarterfinals of the Miami Masters, losing to 19th seed Jan-Michael Gambill. Along the way, Gaudio dismantled fifth seed Russian Yevgeny Kafelnikov, and toughed out a three-set slugfest against future French Open winner and 12th seed Juan Carlos Ferrero. Although he did not manage to win his first title in 2001, Gaudio had some success, making a final, a semifinal and four quarterfinals (one of them at the prestigious Miami Masters). In addition to this, he helped Argentina return to the World Group with a perfect 5–0 record in his singles matches, which were all played in Argentina on clay courts. The year was not great though; Gaudio lost a lot of early-round matches and an astounding 12 first-round matches, never making it past the first round of a Grand Slam. Because of his inability to win these early-round matches, Gaudio's ranking slipped from No. 34 at the beginning of the year to No. 48 at the end of 2001.