Alexander Martin Sullivan (1829 – 17 October 1884) was an Irish Nationalist politician, barrister, and journalist from Bantry, County Cork.
Deeply influenced by the distress he then witnessed, he afterwards joined the Confederate Club formed at Bantry in support of the revolutionary movement of the Young Irelanders, and was the organiser of the enthusiastic reception given by the town to William Smith O'Brien in July 1848 during the insurgent leader's tour of the southern counties.
From 1861 to 1884, in conjunction with his elder brother, T. D. Sullivan, he made The Nation one of the most potent factors in the Irish Nationalist cause, and also issued the Weekly News and Zozimus.
As a member of the Dublin Corporation, he secured a magnificent site for the Grattan Monument, towards which he donated £400, the amount of a subscription by his admirers while he was undergoing imprisonment for a political offence in 1868.
His last great case was on 30 November 1883 when he was a colleague of Lord Russell in the defence of Patrick O'Donnell for the murder of James Carey, an informer.