- MARLE (Marine Liaison Element)
- MARLO (Marine Liaison Officer)
- MAROP (Marine Operators)
- MARP (Michigan Association of Railroad Passengers)
- MARPOL (International Convention For The Prevention of Pollution From Ships)
- MARRA (Scratch, Dent, Deface.)
- MARRES (Manuel Radar Reconnaissance Exploitation System)
- MARRS (Martyn Young, Alex Ayuli, Rudy Tambala, Russell Smith, and Steve Young (British electronic music act))
- MARS (Military Amateur Radio Station)
- MARSA (Military Assumes Responsibility For Separation of Aircraft)
- MARSEC (Maritime Security)
- MARSOC (Marine Corps Special Operations Command)
- MARSOF (Marine Corps Special Operations Forces)
- MART (Mid- Atlantic Rail Trail)
- Mart., M., MM. (Martyr, Martyres ("Martyr", "Martyrs" Breviary))
- MARTA (Multiple Access RouTing Authority)
- MARV (Mini Autonomous Robot Vehicle)
- MARVEL ((project) Multi-object Apache Point Observatory Radial Velocity Exoplanet Large-area Survey, a NASA-funded project to search for exoplanets)
- MAS (NATO Military Agency for Standardization)