The United Nations Marine Corps (UNMC) is the ground forces branch of the United Nations military. Well trained and equipped, they serve on Earth and aboard UNN ships and stations. Like their Martian counterparts, UN Marines use powered, vacuum-rated armor, operate in a variety of combat environments, and utilize a variety of weapons and tactics.
A UN Marine detachment guards the UN border beyond the Martian embassy on Earth whom Gunnery Sergeant Bobbie Draper surrenders to upon escaping from the embassy, requesting political asylum on Earth.
A UN Marine outfitted with Reaver Powered Armor is posted to stand guard outside the med-bay of the UNN Agatha King by Fleet Admiral Augusto Nguyễn after he orders Cotyar Ghazi detained there as a prisoner of war. He is later replaced by a UNN MP after Lieutenant Shaffer orders him to abandon his post to assist in combating a supposed electrical fire to allow Admiral Souther to access the med-bay and speak to Cotyar in private
UN Marines are later deployed to the OPAS Behemoth from the UNN Thomas Prince and other UNN ships trapped in the Slow Zone by Commander Kunis to maintain order and security aboard the OPA vessel in anticipation of an outbreak of mass hysteria after Commander Klaes Ashford convinces the UNN and MCRN commanders that they must use the Behemoth's communications laser to destroy the Ring to prevent the Ring Station from destroying the Sol system which would strand the flotilla as a result.