The 54-year-old guided the Tigers to a 13th-placed finish in the Championship in his first and final full season with the club
Nigel Adkins will leave Hull when his contract expires at the end of the month.
The ex-Tigers boss guided the Championship club to a 13th-placed finish in his first and final season at the KCOM Stadium.
Adkins was appointed Hull boss in December 2017 following the sacking of Leonid Slutsky.
In a statement released on social media, Adkins revealed he rejected a contract extension from Hull chiefs.
He said: “It has been an honour and privilege to be head coach of Hull City since I joined the club in December 2017. I would like to thank Assam and Ehab Allam for the opportunity to manage Hull City and also for the offer of a contract extension.
“However, it is with a heavy heart and after much careful consideration that I have decided that our futures are not aligned and that I will be leaving the club when my current contract finishes on the 30th [of] June.
“I have really enjoyed me time in Hull and have met and worked with some fantastic people. In the first six months at the club we managed to provide stability, retaining our place in the Championship.
“In our first full season we progressed to finishing 13th following an outside push for promotion.
“I’d like to thank all the staff and players, and in particular Andy Crosby, my assistant, for the hard work, camaraderie, togetherness and support they have shown me and wish them all good luck with their future careers.
“The unity the supporters have shown with the staff and players has also been very special. The fantastic relationship I have had with the fans makes leaving extremely difficult.
“I will take with me very special memories from the past 18 months. Together everyone achieves more. Nigel.”