Alun Cairns is "a great candidate" for the Vale of Glamorgan, the prime minister has said on a visit to Deeside.
Boris Johnson backed the former Welsh secretary who resigned on Wednesday after he became embroiled in a row about the collapse of a rape trial "sabotaged" by a former aide.
Mr Johnson said Mr Cairns had been right to step down.
He denied that his general election campaign had been thrown off course.
Mr Cairns had resigned pending an investigation into whether he breached the ministerial code.
Mr Johnson, visiting Iceland headquarters on Deeside Industrial Estate, said: "Alun has resigned from the cabinet, which is quite appropriate while the investigation takes place.
"He's quite right to stand. He's a great candidate... and we'll be backing him all the way."
Mr Cairns earlier declined to answer questions about when he knew about the trial's collapse.
Ross England had been selected to stand as a Welsh assembly candidate for the Vale of Glamorgan last December - eight months after evidence he gave in court led to the collapse of a trial.
Mr Cairns had denied being aware of Mr England's involvement in the collapse until last week, but resigned after BBC Wales obtained an email about the case that was sent to him in August 2018.
The former minister has faced calls from the rape victim, Labour, the Liberal Democrats and Plaid Cymru to quit as a candidate.
Dismissing suggestions the resignation had hit his campaign, Mr Johnson said: "I think what the people of this country can see is that Parliament is just paralysed, we can't get Brexit through, Wales voted Leave.
"People in Wales want us to get this thing done. That's what we're going to do. We have it ready to go.
"We have a great deal. We can get it through by January, get it back into the House of Commons before Christmas, get out by the end of January and then devote ourselves to the priorities of the British people."
Asked if the Welsh Conservatives have a problem with women, Mr Johnson said there were "fantastic female candidates" for the Tories "across the whole of the UK".
"We are going to be campaigning for an amazing future for this country, for women, for men."
Mr Cairns' former colleague Mr England told a rape trial in April 2018 that he had been in a casual sexual relationship with the victim - despite such evidence being ruled inadmissible. She has denied the claim.
Mr England was thrown out of court by the judge and the trial collapsed. The defendant James Hackett was subsequently convicted of rape at a retrial.
Eight months later Mr England was selected to stand in the Vale of Glamorgan for the Welsh Assembly.
Other candidates standing to be the Vale of Glamorgan MP in the 12 December general election include Belinda Loveluck-Edwards for Welsh Labour and Anthony Slaughter for the Wales Green Party.
The close of nominations is 14 November.