Humberside Police Chief Constable Justine Curran has stepped down 18 months ahead of her retirement.
The announcement comes after the force was told by Her Majesty's Inspectorate of Constabulary (HMIC) in November that it "requires improvement".
Ms Curran, who was due to retire in September 2018, said the "time is right" to now leave.
Last May, Police and Crime Commissioner Keith Hunter gave her six months to convince him "that there is a plan".
Mr Hunter said: "It is a mark of her commitment to Humberside Police and the public it serves that she has agreed to make way for a new chief constable who can tackle the continuing challenges the force faces, including those identified by HMIC in their reports over the last few years.
"As police and crime commissioner, I wish her well and thank her for her service to the force."
'Time is right'
Ms Curran, who was appointed as the force's first female chief constable in 2013, said: "It was not an easy decision to leave something that I am passionate about and have given huge commitment to.
"I am proud of the significant impact we have achieved, working every day to make Humberside safer and stronger. Humberside remains a very safe place to live, work and visit.
"However, I feel the time is right for the next chapter and having discussed this with the police and crime commissioner, who is about to launch the new Police and Crime Plan, I feel it is time to pass on the baton and let someone else lead the force to deliver the aims of this plan."
She added: "I would like to thank my colleagues past and present, officers and staff for their commitment and support."
Deputy Chief Constable Garry Forsyth has been appointed as the new interim boss.
Sheffield-born Ms Curran served as an officer in the Greater Manchester and Merseyside forces, before becoming Tayside chief in 2009.
No financial settlement had been made for her early retirement.