M78 or M-78 may refer to:
- BMW M78, a straight-six engine built by BMW
- M-78 (Michigan highway), a state highway in Michigan
- Messier 78, a reflection nebula in the constellation Orion
- E-M78 also known as E1b1b1a, a Y chromosome haplogroup
- M78, or the Land of Light: The Homeland of Ultraman and his family
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