Dundee West MSP Kate Maclean is to stand down at the next election.
Kate Maclean had already been selected to fight the election
She will not contest the seat in the Holyrood polls next May, despite already being selected to do so by Scottish Labour.
In a letter to party members, the former leader of Dundee City Council said she had had "a change of heart".
Scottish Labour leader and First Minister Jack McConnell said he was "very disappointed" with Ms Maclean's decision to step down.
Local party members in Dundee will decide on a replacement candidate at their next meeting, a process likely to take 10-12 weeks.
'Still backing Labour'
Ms Maclean's decision to quit the Scottish Parliament has come after two other Labour figures, Glasgow Rutherglen MSP Janis Hughes and Highlands and Islands MSP Maureen Macmillan announced they would not be standing for re-election.
Ms Maclean, 48, who has represented Dundee in the Scottish Parliament since 1999, declined to be drawn on her decision to leave, but insisted she was still "100%" behind Labour and Jack McConnell.
"I've spent 19 years as an elected politician and I feel it's time for a change," she said.
"I'm not leaving because of any one thing, it's for a variety of different reasons."
"If I want to change what I'm doing then I'd rather do it while I'm in my 40s and not when I'm in my 50s."
Ms Maclean, born in Dundee in 1958, said she had yet to decide on her new career path, but said she would remain a Labour member.
"One of the things that standing down from Holyrood will allow me to do is get back to being more active in the grassroots of the party," she added.
Commenting on Ms Maclean's announcement, Mr McConnell said: "I am very disappointed although I respect her decision.
"Kate has been an outstanding representative for the whole city of Dundee for many years, both as an MSP and as council leader."
Ms Maclean was first elected as MSP for Dundee West with a majority of just 121 over the SNP.
In 2003 she increased her majority to 1,066, and in last year's Westminster election Labour had a majority of 5,379, again over the SNP.
The Nationalists, who hold the Dundee East seat in both the Westminster and Scottish parliaments, are certain to make Dundee West one of their top targets next year.