William Morgan Jellett (19 May 1857 – 27 October 1936) was an Irish Unionist Member of Parliament (MP) in the Parliament of the United Kingdom. The Irish Unionists were the Irish wing of the Conservative Party. He was the son of Rev. John Hewitt Jellett, Provost of Trinity College, Dublin and was born in Dublin.
He attended Trinity College, Dublin, before being called to the Irish Bar in 1882. He became a Queen's Counsel (QC) in 1899. He was private secretary to Lord Ashbourne, the Lord Chancellor of Ireland 1885-6, 1886-1892 and 1895-1905.
Jellett was MP for Dublin University 28 July 1919-1922, having been defeated in the constituency at the 1918 general election. He was the last United Kingdom MP to have been elected in the twenty six counties which became the Irish Free State.
Jellett left the House of Commons in 1922 when his constituency ceased to be represented in the United Kingdom Parliament.