- 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)
