Barnsley MP Dan Jarvis has been chosen as the Labour candidate for the role of Sheffield City Region mayor.
Mr Jarvis, 45, was elected with 2,584 votes, beating rival, city councillor Ben Curran, who got 1,903.
A ruling by the party's National Executive Committee means if he wins the mayoral race he will have to stand down as MP.
May's ballot will see voters choose South Yorkshire's first directly elected mayor.
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Mr Jarvis said he was "proud to have been chosen" and "grateful for the opportunity to serve".
He added: "The election of mayor comes at a pivotal moment for the Sheffield City Region.
"To make the most of new opportunities, our first mayor will need to work with both local and national government to negotiate the best possible deal for the people of South Yorkshire."
Former soldier Mr Jarvis had planned to retain his parliamentary seat and use the platform to campaign for a wider devolution deal covering the whole of Yorkshire.
However, Labour's National Executive Committee ruled this week that no-one could serve in two elected roles.
The Labour Party confirmed that ruling would only apply if Mr Jarvis was elected as mayor.
In a tweet, the other candidate Mr Curran congratulated Mr Jarvis and said the task now was to "ensure a Labour mayor for South Yorkshire is elected on 3 May".
Analysis by James Vincent, BBC Look North political editor
Decisions, decisions, decisions.
Dan Jarvis' selection as Labour's candidate creates a dilemma from the Barnsley MP – and perhaps a bigger problem for his party.
As voting was wrapping up in the nomination process Labour decided to pass a rule that told its MPs they couldn't have any other major elected role at the same time. Odd timing as that only applied to one of the candidates. But now Mr Jarvis faces having to leave the House of Commons if he becomes mayor – or stay as an MP and turn down the mayor's job.
There is of course a third option. Dan Jarvis has always said he wanted to do both jobs – at least for two years while he moulds South Yorkshire devolution into a plan involving the whole county. I'm not sure he'll readily accept having to stand down.
This all could become a big mess for Labour.