Sheen, 66, who recently finished seven seasons playing President Josiah Bartlett on the Emmy-winning TV series, registered at National University Galway in western Ireland, where he plans to study philosophy, English literature and oceanography as part of a bachelor of arts degree.
Sheen, who previously said he did not want to be a distraction for the 15,000 students at the Galway university, declined to comment and told university officials he wanted his arrival to be as private as possible.
The university president, who greeted Sheen as he signed up for his courses, said he expected students to give Sheen enough space to enjoy his new life of learning.
"He's never been to college and wants to come here to be a regular student. It is a great privilege to have him here, but we want to provide him as natural an experience as possible," said Iognaid O Muircheartaigh, who admitted to being a big fan of Sheen's work in "The West Wing" and in the 1979 Vietnam epic "Apocalypse Now."
"I'm sure there will be an initial flurry of excitement as classes full of students, mainly 18 to 21 years of age, see who's in the seat beside them, but in a short time I'm sure he will fit right in. We Irish try not to make too much of a fuss about celebrities. We seem to recognize they're just real people," O Muircheartaigh said in a telephone interview.
Sheen has said he considered studying in Galway after meeting O Muircheartaigh at a university fundraiser in Los Angeles last year, and decided to do it after receiving an honorary doctorate from NUI Galway in March. Speaking at the time, Sheen said he was "very serious" about his hope to become "the oldest graduate" of the 161-year-old university.
He attended high school in his native Dayton, Ohio, but never attended college. Sheen has said he deliberately flunked his entrance exam to the University of Dayton so that he could pursue his dream of acting in New York City instead.
Sheen has a network of relatives in Ireland because of his mother, Mary Anne Phelan, who immigrated to the United States from County Tipperary.