Bishop Derry was 79.
He passed away at Letterkenny University Hospital, where he was receiving treatment, according to the Catholic Communications Office.
Dr Hegarty led the diocese of Derry from 1994 to 2011, when he retired due to an “irreversible and progressive” illness.
Before that, he served as the Bishop of Raphoe.
Said Colum Eastwood: “I was sorry to hear of Seamus Hegarty’s death today and I want to express my sincere condolences.
“I will miss our conversations and the contribution Seamus made to our city.
“He had huge shoes to fill when he succeeded Bishop Daly in Derry and served communities in difficult times.”