Bowe Bergdahl, the US Army Sergeant, has told a military judge he is pleading guilty to desertion and misbehaviour before the enemy.
His lawyer said the prosecution and defence have not agreed to a stipulation of facts in the case, which is an indication that they did not reach a deal to limit his punishment.
Bergdahl is charged with endangering his colleagues by walking away from a remote post in Afghanistan in 2009.
He told the judge that he now understands that what he did caused others to search for him.
The 31-year-old's guilty pleas bring the saga closer to an end eight years after Bergdahl's disappearance in Afghanistan set off search missions by scores of his fellow service members.
President Barack Obama was criticised by Republicans for the 2014 Taliban prisoner swap that brought Bergdahl home, while President Donald Trump harshly criticiced Bergdahl on the campaign trail.
Video captures release of Bowe Bergdahl
Meanwhile, serious wounds to service members who searched for Bergdahl are expected to play a role in his sentencing.
Bergdahl's five years of captivity by the Taliban and its allies are also likely play a role in what punishment he receives.
Bergdahl had himself told a general during a preliminary investigation that he left intending to cause alarm and draw attention to what he saw as problems with his unit. He was soon captured.
The military probe of Bergdahl began soon after he was freed from captivity on May 31, 2014, in exchange for five Taliban prisoners.