The 1948 Armagh by-election was held on 5 March 1948, following the death of Ulster Unionist Party Member of Parliament William Allen.
Allen had held the seat of Armagh since its recreation for the 1922 UK general election. He had often been elected without a contest; the last election at which he had faced an opponent was in 1935, where he had taken 67.6% of the vote against Charles McGleenan, an independent Irish republican candidate.