| Mariner West Pipeline Project | Sunoco Logistics | PA | | |
| Marion/Delaware Cos Extension | Suburban Gas Co | OH | In service 2005 | |
| Maritech Resources coastal use permit P20060174 pipeline | Maritech Resources, Inc. | LA | | |
| Maritech Resources coastal use permit P20070666 pipeline | Maritech Resources, Inc. | LA | | |
| Maritimes & Northeast 2001 Expansion | Maritimes & Northeast Pipeline | ME | In service 2001 | |
| Maritimes & Northeast Phase II (ME) | | ME | In service 1999 | |
| Maritimes & Northeast Pipeline PF08-17 | Maritimes & Northeast Pipeline | | | |
| Maritimes & Northeast Pipeline Phase III Project | Maritimes & Northeast Pipeline | MA | In service 2003 | |
| Maritimes & Northeast Pipeline, L.L.C. (Ph II) (Phase II - Joint Facilities) (Capacity allocated to the respective Maritimes and Portland Projects | Portland Natural Gas Transmission System | ME | In service 1998 | |
| Maritimes & Northeast Veazie Lateral | | ME | In service 1999 | |
| Maritimes Phase II | Maritimes & Northeast Pipeline | ME | | |
| MarkWest Arkoma Connector Pipeline Project | MarkWest Pioneer LLC | OK | 2009 | |
| MarkWest Lea County Expansion | MarkWest New Mexico LP | NM | In service 2008 | |
| MarkWest Pinnacle "Hobbs" Lateral Expansion | Pinnacle Pipeline Company | NM | In service 2004 | |
| MarkWest Utica EMG - Seneca to Cadiz Phase 2 South | | OH | | |
| Market Expansion Project | Discovery Gas Transmission LLC | LA | In service 2005 | |
| MarketAccess Project | Iroquois Gas Transmission System, L.P. | CT, NY | In service 2008 | |
| Markham Booster Station Project | Gulf Shore Energy Partners, LP | TX | | |
| Marksville Compressor Station - M100 Installation Project | Gulf South Pipeline Company, LLC | LA | 2021 | |
| Markwest Energy PF08-2 | Markwest Energy Partners, L.P. | | | |
| Marquette Branch Line Expansion Project | Northwest Natural Gas Co | MI | 2018–2019 | |
| Marquette Connector Pipeline Project | Semco Energy Gas Company | MI | | |
| Marquette-Ishpeming | Mid America/Northern Natural Gas | MI | | |
| Marshall County Mine Panel 17W Project | Texas Eastern Transmission, LP | WV | 2017–2018 | |
| Marshall County Mine Panel 18W Project | Texas Eastern Transmission, LP | WV | 2018–2019 | |
| Marshall County Mine Panels 19E and 20E Projects | Texas Eastern Transmission, LP | WV | In service 2022 | |
| Marshalltown Pipeline | | IA | | |
| Marshfield Reduction Project | ANR Pipeline Company | WI | In service 2012 | |
| Mary Patterson LP Pipeline-Reissue | Antero Midstream LLC | WV | | |
| Marysville Connector Pipeline | Columbia Gas of Ohio | OH | | |