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. Several Marine MPs posted in the UNN dreadnought's CIC later assist Nguyễn's forces in suppressing Admiral Souther's mutiny aboard the Agatha King and detaining many other CIC personnel who refuse to comply with Nguyễn's orders when he orders the Agatha King to fire on UNN ships refusing to follow his orders. Several of the Marine MPs remaining in the CIC are later killed in a confrontation with Lieutenants Shaffer and Mancuso after Nguyễn destroys the UNN Jimenez.
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.