| M3OC | Tesla Model 3 Owners Club 
| Others |
| M3P | Microsoft Multimedia Productions
| Others |
| M3Q | Mazda 3 Quebec Club
| Others |
| M3RC | Municipal Resource Recovery & Research Collaborative
| Others |
| M3T | Bmw M3 Touring
| Others |
| M3T2 | Muti-Mission Medium Tactical Transport
| Others |
| M3U | MP3 URL
| Others |
| M3U | MPEG URL Sound reference file
| Others |
| M3UA | Mtp3 User Adaptation
| Others |
| M3V | Mobile Medical Monitoring Vehicle
| Others |
| M3W | Mechanized Three Wheeler
| Others |
| M3W | Morgan Three Wheeler
| Others |
| M3WC | Morgan Three Wheeler Club
| Others |
| M4 | ICH M4 Expert Working Group (EWG) [focusing on Common Technical Documents for sections of the NDA]
| Others |
| M4 | M4 preprocessor file (unix)
| Others |
| m4 | Macro processor version 4
| Others |
| M4 EWG | ICH M4 Expert Working Group
| Others |
| M40 | Monroe County Airport, Aberdeen, Mississippi USA
| Others |
| M4A | Medicare for all
| Others |
| M4A | Mpeg4 Audio
| Others |
| M4C | Bmw M4 Competition
| Others |
| M4C | Meet For Coffee?
| Others |
| M4CS | Motorsport Four Series Club Sport
| Others |
| M4IF | Mpeg4 Industry Forum
| Others |
| M4L | Montana 4wd Luxury
| Others |
| M4M | Male For Male
| Others |
| M4OD | Manual Four Speed Overdrive
| Others |
| M4P | Maserati Quattroporte
| Others |
| M4S | Mid-Engine Four-Cylinder Sport
| Others |
| M4SIZ | Emphasize
| Others |
| M4V | Mpeg4 Video
| Others |
| M4W | Mad 4 Wheels
| Others |
| M4W | Motorized Four Wheeled
| Others |
| M4Y | Mortgage 4 You
| Others |
| M5 | Manual Five Speed
| Others |
| M50 | Mean of 1950 Coordinate System
| Others |
| M577 | Model 577 Truck; Carrier, Command Post, Light Armored
| Others |
| M5C | Bmw M5 Competition
| Others |
| M5CS | Bmw M5 Clubsport
| Others |
| M5K | Maserati 5000
| Others |
| M5K | Mobile 5000 Mile
| Others |
| M5OD | Manual Five Speed with Overdrive
| Others |
| M5T | Motorsport Five Series Touring
| Others |
| M6GC | Motorsport Six Series Gran Coupe
| Others |
| M6OC | Mazda 6 Owners Club
| Others |
| M7 | Sound (Farallon's MacRecorder format) (sampling rate 7k)
| Others |
| M700 | Model 700
| Others |
| M76 | Picayune Airport, Picayune, Mississippi USA
| Others |
| M7B | Mark Seven Blass
| Others |
| M7C | Mini Seven Club
| Others |