Monica Lennon has confirmed she will stand in the Scottish Labour leadership contest.
The Central Scotland MSP joined the race late on Sunday. Candidates had until midnight to declare their intention to run.
Glasgow MSP Anas Sarwar has also launched a bid to succeed Richard Leonard, who resigned on Thursday.
Ms Lennon, 40, previously said Scottish Labour must split from the main UK party to end its "losing streak".
She said Scottish Labour was still treated as a "branch office" by London and she warned that "epic change" was needed to avoid further decline.
On Sunday, she tweeted: "Following discussions with party members, I have decided to put my name forward to lead the Scottish Labour Party.
"Our members deserve to have their say about the best way to take forward our vision for a fairer and more equal Scotland. I look forward to being considered."
Ms Lennon, the party's health spokesperson in the Scottish Parliament, led a campaign against period poverty, which saw the unanimous passing of the Period Products (Free Provision) (Scotland) Bill in November.
Mr Sarwar said he looked forward to a "friendly and positive campaign".
He added: "Now more than ever, our country needs political leadership that will bring people together. I want to rebuild Scottish Labour, and then rebuild Scotland."
In an article for the Observer online, Mr Sarwar said: "I firmly believe we cannot go back to society as it was before the pandemic - insecure work, hollowed-out public services, an underfunded health service, and the constant focus on another independence referendum when there's far more important things we need to be dealing with."
In January last year, Ms Lennon wrote in the Daily Record that the party could still transform itself into a "modern, dynamic political force" despite its disastrous electoral performance over the past decade.
Ms Lennon said: "Scottish Labour needs to stand or fall by its own decisions. We either continue at the mercy of the UK party's distant structures or we become a party in our own right.
"This would mean raising our own funds, persuading trade unions to affiliate and reaching out to voters and organisations in a more meaningful way.
"Our MPs would take the Scottish Labour whip and vote with our sister party at Westminster, if persuaded."
The contest is taking place just a few months ahead of the Scottish Parliament election, which is scheduled to be held on 6 May.
The party's executive agreed to a condensed timescale for any contest with the Holyrood elections less than four months away.
The last Scottish Labour leadership contest in 2017 - when Mr Leonard defeated Mr Sarwar - took two-and-a-half months.
With the deadline to declare intention to run now past, candidates will require support from at least four of the party's MSPs or its sole Scottish MP by midday on Tuesday to be formally nominated.
A leader will be declared on 27 February.