Alliance Party leader David Ford is to resign after 15 years.
Although he will continue serving as an MLA for South Antrim, the former justice minister said the time was right to stand aside.
He said: "With my 15th anniversary, I felt it the most appropriate time to step aside and pass the reins to a new generation of leadership for the party."
Mr Ford's resignation as party leader takes effect from noon on Thursday.
His successor will be chosen at a special Alliance Party council later this month.
The father of four added: "Whoever succeeds me as Alliance leader will be taking over a party on the rise.
"I am confident they will continue that upwards trend of growing the party, while providing leadership for everyone in our community."
"Alliance has a well-established new Assembly team, which has hit the ground running by successfully submitting a number of private member's bills and continuing to provide the effective scrutiny of the Executive others have so far failed to do.
"I am proud to represent the people of South Antrim, who most recently re-elected me for my fifth Assembly term in May, and I will continue to do that in Stormont and in the constituency."
The tenure of Mr Ford, who is an ex-social worker, has been the longest in the Alliance Party's history.
He assumed the leadership role in 2001 when Sean Neeson stood down in the wake of poor election results.
Six years later, the party achieved one of its highest ever vote shares and made further gains in the 2011 Assembly elections.
Mr Ford was appointed Northern Ireland's first justice minister in almost 40 years in 2010 - becoming one of two Alliance Party ministers to serve in the five-party coalition Executive during the last mandate.
During that time he became embroiled in a bitter dispute with the legal profession over plans to slash the legal aid bill.
In 2012 his party endorsed gay marriage.
And in February of this year he brought forward controversial legislation which would have legalised abortion in cases of fatal foetal abnormality.
Following May's election, Mr Ford said his party would not be in a position to accept the justice ministry.
Among the favourites to take on the role will be current deputy leader and former E ast Belfast MP Naomi Long.
Nominations will close next week and t he successful candidate must receive more than 50% of the votes cast by members of the Alliance Party's 250-strong council during a specially convened meeting on October 26.
In a tribute, Mrs Long said the outgoing leader had navigated some difficult times.
She said: "His leadership over the last 15 years has transformed the party and made a huge contribution to peace and stability in Northern Ireland.
"It has been a pleasure to serve as his deputy leader for the last 10 years and to work closely with him to shape and modernise the party."
Former employment minister Stephen Farry said Mr Ford has left a "powerful legacy".
Political rivals also praised Mr Ford.
Mike Nesbitt, leader of the Ulster Unionists, said: "I congratulate David on sticking the pace for 15 years as party leader - that's quite a shift in what can be a very lonely and exposed position.
"He led his party during some very challenging times both for Northern Ireland and for these institutions and I am sure he will be remembered for his contribution."
SDLP leader Colum Eastwood said: "He led his party with composure and resilience particularly in the face of violence following the flag protests.
"His dedication to democracy and the rule of law has served him well."