Seosamh Mac Grianna (20 August 1900 – 11 June 1990) was a writer from County Donegal. He was born into a family of poets and storytellers, which included his brothers Séamus Ó Grianna and , in Rann na Feirste, County Donegal, at a time of linguistic and cultural change. Mac Grianna is the most high-profile modern writer in Ulster Irish.