John Williams, 28, entered his plea in a Portland courtroom, according to the Bangor Daily News.
Williams allegedly shot and killed Corporal Eugene Cole at the end of April. Williams then stole the officer's cruiser and robbed a local gas station before running off.
Williams was found hiding in the woods and taken into custody after a massive four-day manhunt.
Officials from the Maine Attorney General's Office said they will be seeking the maximum sentence for Williams.