Julian Knowle (born 29 April 1974) is an Austrian former professional tennis player. Being a born left-hander, Knowle was one of the few on the ATP Tour who played his forehand, backhand, and even volleys double-handed. He was Austria's most successful doubles player in history by reaching world No. 6 in the ATP doubles rankings in January 2008, before being matched by Jürgen Melzer, who reached No. 6 in September 2010, and overtaken only by Alexander Peya, who reached No. 3 in August 2013.

Knowle was a successful player on the ATP Challenger Series, winning the Challenger tournaments in Kyoto (1999), Caracas (2001), Graz (2001), and Andrezieux (2002), and reaching the finals in Yokohama (2000), Bristol (2000), Besançon (2000), and Graz (2003). He also won several Futures tournaments. Knowle's best ATP singles ranking was world no. 86 in July 2002. His final appearance in the main draw of a singles tournament was in the Graz Challenger in 2005 where he reached the quarterfinals.

Knowle reached his first of two Grand Slam finals at Wimbledon in 2004 with Nenad Zimonjić of Serbia. Eventually, the team was defeated in four sets by Jonas Björkman and Todd Woodbridge. The only Austrian to reach a final at Wimbledon before was Georg von Metaxa in doubles in 1938, where he too lost.

In 2005 Knowle teamed up with Czech Petr Pála for several months without being able to continue his successful run with Zimonjić. This changed when he formed a team with fellow Austrian player and left-hander Jürgen Melzer, joining him throughout most of 2005 and 2006. Together, they won two tournaments in doubles and reached another five finals.

Following Melzer's hand injury in early 2007, Knowle found a new partner in Simon Aspelin of Sweden.

At the 2007 US Open, seeded tenth with Aspelin, Knowle achieved the greatest triumph of his career by winning the tournament, his first Grand Slam. In the first two rounds, they won over Kubot/Skoch and got a walkover over Calleri/Horna. They went on to upset eighth seeds Jonathan Erlich and Andy Ram (who would go on to win the 2008 Australian Open men's doubles) in the third round. In the quarterfinals, they shocked the top seeds Bob and Mike Bryan, having lost to them only weeks before. In the semifinals, they held off unseeded Julien Benneteau and Nicolas Mahut, 7–6⁽²⁾, 1–6, 6–3, before winning the final 7–5, 6–4 over the ninth seeds, Pavel Vízner and Lukáš Dlouhý. They had previously won three tournaments together. This win put them into the no. 5 position in the ATP Doubles Race, and also gave Knowle his first top-10 ranking in doubles.

Knowle was the second of so-far four Austrian tennis players to win a Grand Slam tournament (the first in doubles). The first Austrian to win a Grand Slam tournament was Thomas Muster at the 1995 French Open; the third was Jürgen Melzer, who won the 2010 Wimbledon Championships – Men's doubles and later the 2011 US Open – Men's doubles with his German partner Philipp Petzschner as well as the 2011 Wimbledon Championships - Mixed doubles with his later wife Iveta Benešová; the fourth was Dominic Thiem, who won the 2020 US Open - Men's Singles tournament.

Their excellent first year as a team enabled Knowle and Aspelin to participate in the Tennis Masters Cup in Shanghai, China for the first time. They surprisingly made it all the way to the final, beating Pavel Vízner and Lukáš Dlouhý, Arnaud Clément and Michaël Llodra, and finally Martin Damm and Leander Paes, before eventually falling in straight sets to Mark Knowles and Daniel Nestor 2–6, 3–6.