Sheen, 60, who plays the US president in hit TV series The West Wing, was also fined $500 in Los Angeles on Wednesday.
Sheen was arrested in October at the Vandenberg Air Force Base in California. He was part of a demonstration against the proposed US "Star Wars" military nuclear defence system.
Sheen said he had to plead guilty because of work obligations but would still oppose "Star Wars".
"I have a prior engagement with The West Wing, another year on my contract, and I can't risk losing (at a trial) and going to prison for six months.
"Morally, I'm bound to fulfil my contract. That was the whole motivation.''
Under the terms of his probation, Sheen is banned from Vandenberg without permission from officials there.
The actor said his previous federal court experience had left him "conquered, but unafraid," and he would continue to oppose "Star Wars" initiatives.
Outspoken
A spokesman for the US Attorney's Office in Los Angeles said most of the other protesters arrested with the actor had pleaded innocent to trespassing charges and would face trial later this year.
Earlier in the day, Sheen joined his fellow protesters outside court, who were waving banners saying: "No Star Wars".
Sheen said: "I can't think of a worse thing to inflict on the universe than nuclear weapons in outer space, and all of them pointed to the Earth."
Sheen, also the star of movies Apocalypse Now and Wall Street, is famous for his outspoken and proactive political views.
He has been arrested 70 times for his involvement in protests for issues such as nuclear disarmament and homelessness.
Sheen, a prominent Democrat Party supporter, has also made known his opposition to US Republican president George W Bush.
In an interview with the Radio Times, Sheen said: "George W Bush is like a bad comic working the crowd, a moron, if you'll pardon the expression."