Ieuan Wyn Jones has revealed he will stand down as Plaid Cymru leader in the first half of the Welsh assembly's five-year term.
He said it had always been his intention to leave the role at some point before the next election in 2016.
The announcement follows a poor showing for Plaid at last week's election when it lost four seats.
Former Plaid president Lord Elis-Thomas has said he would allow his name to go forward as a leadership candidate.
Speaking at Beaumaris in his Anglesey constituency, Mr Jones said it had been a great honour to lead the party over the past 11 years.
The timetable for his departure means Mr Jones will go some time in the next two-and-a-half years.
In a statement delivered in front of party colleagues, he said the "time was right" for him to make his plans clear.
He said it had been an honour to lead the party and to serve as deputy first minister in the coalition with Labour in the last assembly.
"I have also witnessed many historic and momentous events during that period, not least leading the party into government for the first time in its 86-year history and the referendum on law-making powers," he said.
Mr Jones was MP for Ynys Mon from 1987 to 2001 and has been the island's AM since the first assembly in 1999. He first took up the party leadership in 2000.
Last week's election saw Plaid slump to 11 seats, its worst tally since the assembly was established.
There has been criticism that the party's campaign was too negative by attacking its former coalition partners in Labour.
Mr Jones said the result was a "disappointment", adding: "As leader I take my share of the responsibility for those results.
"The party obviously needs time to reflect on the results, look long and hard at our message, our party structures and campaigning abilities."
Lord Elis-Thomas said if Plaid activists in his Dwyfor Meirionnydd constituency want his name to go forward as a candidate for the leadership he would give them a "straight answer".
"I am afraid that they have decided to ask me the question," he told BBC Radio Wales.
Lord Elis-Thomas, who was leader of Plaid between 1984 and 1991, will speak to party officials in Porthmadog on Friday night. He ended a 12-year stint as the assembly's presiding officer this week.
Mr Jones said he was confident Plaid would recover "stronger and better, provided we understand the need to change and modernise".
An immediate leadership election would not be in the party's interest because of the need to review the election result, he said.
He thanked his family for their support, including his wife Eirian - "a rock through it all" - and his three children and four grandchildren.
Former Plaid chairman John Dixon, who left the party earlier this year because he was unhappy with its direction, said Mr Jones has made the right decision to quit.
He told BBC Wales his departure was inevitable and that it will be a chance for the party to decide what it stands for.
Plaid Westminster leader Elfyn Llwyd said: "He has shown great commitment and dedication to the party over the years. Wales has come a long way and so has Plaid Cymru."
First Minister Carwyn Jones said: "I worked well with Ieuan Wyn Jones in the four years we were in government together and I wish him well in whatever he does next. I have always known Ieuan to be a decent man and his party will find him hard to replace."