Mountain ash is a name used for several trees, none of immediate relation. It may refer to:
Eucalyptus regnans from Australia, the tallest of all flowering plants
Fraxinus texensis, from Texas
Sorbus aucuparia, also known as rowan.
Sorbus americana, from eastern North America
In North America (mainly U.S.) trees in the subgenus Sorbus subgenus Sorbus are often styled as mountain-ashes to convey their unrelatedness to true ashes.