William John Twaddell (1884 – 22 May 1922) was a Unionist politician from Belfast.
Twaddell was a draper from Belfast who was educated at a Belfast primary school.
He was a Member of Belfast City Council from 1910 and sat as an Ulster Unionist Party member. In November 1921, he and Robert Boyd organised the Ulster Imperial Guards as a paramilitary force of 21,000 men. He was elected to the Parliament of Northern Ireland for Belfast West from the general election of 1921 until he was assassinated on 22 May 1922 by the Irish Republican Army. He was walking in Garfield Street, to his business, a short distance away, and had been followed closely by his assassins.