Sir Ian Macdonald Horobin (16 November 1899 – 5 June 1976) was a British Conservative Party politician.
He was educated at Highgate School and Sidney Sussex College, Cambridge. Horobin served in the Royal Air Force in both world wars. In 1923 he became Warden of Mansfield House University Settlement.
Horobin was Member of Parliament (MP) for Southwark Central from 1931 to 1935, and for Oldham East from 1951 to 1959.
On 29 March 1962, it was announced that Macdonald Horobin was to receive a life peerage. However, on 13 April he withdraw his acceptance and the following month was arrested for indecency. In July 1962, he was sentenced to four years' imprisonment, having been found guilty for paying young boys for sex.