Nicholas Colohan (1806 – 11 February 1890) was an Irish doctor and professor of medicine at Queen's University, Galway from 1849 to 1879.
Colohan was a native of Ballinasloe and related to William and James Colohan, highly respected doctors who practised in that town.
He graduated with a medical degree from the University of Edinburgh in 1828, though without a surgical qualification. Colohan was one of six doctors associated with Galway's Fever Hospital.
He married Sara, daughter of Dr. Thomas Whistler of Oranmore. Their children included: John Colohan, Frances, William and Nicholas, who was the father of Arthur Colohan (1884–1952).