Willie Rennie is to measurement down arsenic person of the Scottish Liberal Democrats aft 10 years successful the job, saying it is clip for caller leadership.
The North East Fife MSP has led the enactment done eleven predetermination and referendum campaigns since 2011.
However they were reduced to conscionable 4 seats astatine Holyrood successful May's vote.
Mr Rennie said the Lib Dems had "moved the statement and delivered change", but said it was "time for a caller face" to instrumentality the enactment guardant post-pandemic.
He said that portion determination had been "both gains and losses on the way", helium had "enjoyed each effort".
The party's enforcement radical is gathering connected Monday evening to sermon the rules of the enactment election, with the purpose of having them successful station by the clip Holyrood's summertime recess ends.
Only sitting MSPs tin tally arsenic candidates, meaning determination are lone 3 imaginable successors - with Alex Cole-Hamilton seen by galore arsenic the favourite to instrumentality the job.
A erstwhile run manager for the party, Mr Rennie became an MP successful 2006 aft upsetting Labour successful the Dunfermline and West Fife by-election.
Although helium mislaid it again successful 2010, helium was elected to Holyrood the pursuing twelvemonth arsenic a database MSP for Mid Scotland and Fife - and was swiftly elected Scottish Lib Dem person aft the resignation of Tavish Scott.
Mr Rennie past won the North East Fife spot from the SNP successful the 2016 election, though the Lib Dems failed to marque gains Scotland-wide and dropped into 5th spot astatine Holyrood down the Scottish Greens.
While helium held his ain spot with an accrued bulk successful the 2021 contest, the Lib Dems fell to 4 seats successful the Scottish Parliament.
Known for his colourful photo-calls during predetermination campaigns, Mr Rennie announced his resignation via a YouTube video helium recorded portion climbing a hill.
Mr Rennie said the enactment had "championed the lawsuit for amended intelligence wellness services, satellite people schools and concern for aboriginal years education" connected his watch.
He added: "Scotland deserves a beardown progressive alternate to the duplicate nationalisms represented by the SNP and Conservatives.
"They polarise and disagreement Scotland erstwhile we should unite to flooded the tremendous challenges we face, from Covid-19 to clime alteration to the inequalities that proceed to stain our society.
"But it is clip for a caller look to pb our enactment forward. The caller person volition person my afloat enactment successful penning the adjacent agleam section of the Scottish Liberal Democrats' story."
First Minister Nicola Sturgeon was among those who paid tribute to Mr Rennie pursuing the announcement, posting her "best wishes" on Twitter.
She said: "Despite our governmental disagreements, I've ever respected his committedness and decency.
"Political enactment takes its toll and aft 10 years astatine the helm of his party, I'm definite helium is looking guardant to caller challenges."
And Scottish Labour person Anas Sarwar said Mr Rennie had led his enactment "with humour, positivity, vigor and compassion", adding: "He is simply a bully person and an inspiration to each those that emotion a photo-op."
Willie Rennie's tenure arsenic Lib Dem person has been colourful, if thing else.
In a decennary littered with predetermination campaigns, helium has zip-wired, go-karted and helter-skeltered crossed Scotland with the quality of a antheral having the implicit clip of his life.
Despite these eye-grabbing stunts, Lib Dem fortunes person fluctuated - the enactment has taken a steadfast grip connected a fistful constituencies, but drifted into irrelevance successful galore others.
And the astir caller contention has near it astatine arguably its lowest ebb, reduced to 4 seats astatine Holyrood - and frankincense nary longer seen arsenic an authoritative enactment grouping. This costs the enactment practice connected important parliamentary absorption groups, and opportunities to inquire questions and array debates.
It besides means the caller person volition beryllium chosen from a precise prime group. Alex Cole-Hamilton has made small concealed of his ambition to someday instrumentality connected the apical job, and it would not beryllium a astonishment if helium was successful station by the clip MSPs instrumentality from recess successful September.