Health Secretary Eluned Morgan will stand to replace Vaughan Gething as Welsh Labour leader and first minister.
Baroness Morgan has passed the threshold of six nominations to enter the contest after Mr Gething stepped down last Tuesday following the protest resignation of four members of his government following questions about campaign donations.
She is the first person who has formally said they will run, with Labour politicians in the Welsh Parliament having until noon on Wednesday to decide who they will back.
Rural Affairs Secretary Huw Irranca-Davies is running with her as Baroness Morgan’s deputy.
After announcing her candidacy, the Welsh Conservatives called for the Senedd to be recalled from summer recess to appoint a new first minister, with voting by the whole Senedd for a new first minister set to take place on 18 September.
Whoever wins the Labour leadership race in Wales is expected to be voted through as first minister as the party holds the most number of seats.
If nobody else is nominated to stand in the Welsh Labour leader race then Baroness Morgan will win uncontested, instead of facing a contest involving Welsh Labour members voting and a leader being announced on 14 September.
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Baroness Morgan said: “I am proud to stand as a candidate to be the next leader of Welsh Labour, driven by a passion to serve the people and reset the relationship between the government and the people of Wales.
“I am delighted to be teaming up with my fantastic colleague, Huw Irranca-Davies, who shares my passion and experience in government and strong sense of public duty. Together, we are committed to putting Wales back on track.
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“In the coming days, we will set out our priorities to improve public services, create better, greener jobs and empower our communities.
“Our focus will be meaningful change and building a brighter future for communities across Wales.”
Mr Gething has said he will stay on until his replacement is appointed.
He made history in March when he became the first black leader of any European country.
However, he continually faces questions about a £200,000 donation to his leadership campaign from a company that had submitted a planning application for a solar farm that required Welsh government approval.
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A subsidiary company of Dauson Environmental Group also received a £400,000 loan from the Development Bank of Wales, which is owned by the Welsh government.
A row then ensued after he sacked minister Hannah Blythyn for leaking phone messages, which she denied and a newspaper then said she was not their source.
Plaid Cymru then pulled out of its co-operation agreement with Mr Gething’s government.
