Jonathan Siegrist (born 27 August, 1985) is an American rock climber who is regarded as one of the world's most prolific sport climbers, and who has redpointed, and made numerous first free ascents, of a large number of sport climbing routes at and above the grade of 9a (5.14d). Siegrist's breadth of experience at the world's most extreme sport climbing grades, means that he is often looked to for guidance regarding the grading of extreme sport routes in America. While principally known as a sport climber, he has repeated some of the world's hardest traditional climbing routes.

Siegrist began climbing at an early age with his father Bob Siegrist, who was an established 5.13 rock climber, but only started climbing seriously in about 2004, then aged 18. In 2009, Siegrist came to national attention when Climbing Magazine awarded him the Golden Piton Award in the "Breakaway Success" category.

In 2014, aged 28, Siegrist climbed his first 9a+ (5.15a) graded route when he redpointed Chris Sharma's famous 2001 sport climbing route, Realization/Biographie. Over the following year, Siegrist became only the fifth person in history to complete the "9a+ Trilogy", adding La Rambla, and Papichulo. In 2017, Siegrist cemented his status at the 9a+ grade, with ascents of Pachamama, Joe Mama and Chaxi, all completed in one six-week trip to the famous Oliana crag in Spain.

In May 2018, Siegrist came to wider attention with only the second repeat of Sharma's groundbreaking route, Jumbo Love, which was Siegrist's first route at the grade of 9b (5.15b). In 2019, he repeated a second notable 9b graded route making the third repeat of La Planta de Shiva in Spain, and making his own first free ascent (FFA) of a 9a+ (5.15a) route with All You Can Eat in the US. Over the next few years, Siegrist would create several other new 9a+ graded routes,[14] and in 2022, he completed his first FA of a 9b (5.15b) route with Event Horizon, one of America's hardest sport climbing routes.

While Siegrist is known as a sport climber, he has repeated some of the world's hardest traditional climbs, including Sonnie Trotter's internationally famous 2007 route, The Path (5.14a R), and the even harder but less repeated route, The Almighty (5.14a/b R), by Ty Mack. In 2016, Siegrist made the first repeat of Tommy Caldwell's 2013 route, Direct Dunn-Westbay (5.14a/b), belayed by his 66-year-old father, Bob.