Stuart Andrew has left the Wales Office to take up the junior defence minister post vacated by Aberconwy MP Guto Bebb.
Mr Bebb resigned as a defence procurement minister earlier this week in protest at concessions to Brexiteers over the Customs Bill.
Mr Andrew, Anglesey-born Conservative MP for Pudsey in West Yorkshire, joined the Wales Office in January.
His replacement as junior Wales Office minister has not yet been announced.
Welsh Secretary Alun Cairns said: "While I am sad to see Stuart leave the department, I know that he will take on the role within the Ministry of Defence with diligence and determination, following in the footsteps of his outstanding Welsh predecessor."