William Timothy Cotter (1866–1940) was an Irish-born prelate who served as the third Roman Catholic Bishop of Portsmouth, England, from 1910 to 1940.

William Timothy Cotter was born in Cloyne, County Cork, Ireland on 21 December 1866.

He was educated at St. Colman's Fermoy; and studied for the priesthood at Maynooth College.

His consecration to the Episcopate took place on 19 March 1905,[2] the principal consecrator was Bishop John Baptist Cahill of Portsmouth, and the principal co-consecrators were Bishop George Ambrose Burton of Clifton and Bishop Peter Amigo of Southwark.

[3] Bishop Cotter died in office on 24 October 1940, aged 73.