| Abbreviation | Definition |
|---|---|
| MRFU | Missouri Rugby Football Union |
| MRFU | Mongolian Rugby Football Union |
| MRMU | Maranatha Reformed Mothers Union |
| MROC | Midwest Region Organizing Committee |
| MRU | Malaysia Rugby Union |
| MRU | Mano River Union |
| MRU | Mill River Union |
| MSCU | Mennonite Savings and Credit Union |
| MSGU | Montana Sustainable Growers union |
| MSU | Malaysian Students Union |
| MSU | McMaster Student Union |
| MTFU | Metal Trades Federation of Unions |
| MTU | Motor and Turbine Union |
| MTUC | Malaysian Trade Union Congress |
| MTUC | Malaysian Trades Union Congress |
| MTUC | Manchester Trades Union Council |
| MTUC | Mauritius Trade Union Congress |
| MU | Maritime Union |
| MU | Monetary Union |
| MU | Musician's Union |