Denis Olegovich Istomin (born 7 September 1986) is a former Uzbekistani professional tennis player. He reached a career-high ATP singles ranking of world No. 33, achieved on 13 August 2012 and a best doubles ranking of No. 59, reached on 8 October 2012. His most notable result is defeating second seed and defending champion Novak Djokovic at the 2017 Australian Open.

Istomin won two ATP Tour singles titles, at the 2015 Nottingham Open and 2017 Chengdu Open.

Denis Istomin was born in Orenburg to Russian parents Oleg Istomin and Klaudiya Istomina. His father moved the family to Tashkent, Uzbekistan, when Denis was 3 months old. Denis is coached by his mother, who introduced him to the sport at an early age. Injuries resulting from a car crash in 2001 en route to an event kept Istomin away from tennis for 2 years, with doctors initially stating that he would never hold a racquet again. Despite this, Istomin resumed training in April 2003.

He is good friend with Varvara Lepchenko, who previously played for Uzbekistan.

Istomin is one of the few top ATP Professional players, along with Janko Tipsarević and Chung Hyeon, to wear glasses while playing (most players either have near perfect eyesight or wear contact lenses).

Istomin broke into the top–200 late in 2005 and finished the year at No. 196. He won his first Challenger tournament in Bukhara.

Istomin received the Asian wildcard into the 2006 Australian Open, where he played world No. 1 Roger Federer. Federer won in straight sets. He spent much of 2006 in the top 200, reaching a career-high of No. 186 on 1 May, and finished the year at No. 200 in singles.

Istomin was slightly less successful in 2007, spending the entire year outside the top 200 in singles, but rallying towards the end of the year to finish at No. 230, again claiming the Asian Wild card for the 2008 Australian Open. In any case, he won two Challenger titles, Karshi and, for a second time, Bukhara. He reached his career-high of No. 157 on 18 June.