It has smooth bark, lance-shaped leaves that are paler on the lower surface, flower buds in groups of seven to eleven, white flowers and cup-shaped to bell-shaped fruit.Eucalyptus deanei typically grows as a straight forest tree, growing a height of 40–65 m (131–213 ft) with a trunk diameter of up to 2 m (6 ft 7 in) at breast height.

[2][3][4][5][7] Mountain blue gum was first formally described in 1899 by Henry Deane and Joseph Maiden who gave it the name Eucalyptus saligna var.

[8][9] In 1904, Deane raised the variety to species status as E. deanei and published the change in Proceedings of the Linnean Society of New South Wales.

[11] This eucalypt has been classified in the subgenus Symphyomyrtus, Section Latoangulatae, Series Transversae (eastern blue gums) by Ian Brooker and David Kleinig.

[5] It is a dominant tree of tall forests in sheltered valleys where there is plenty of moisture, on clay or loam soils, and alluvial sands, although it sometimes grows on more elevated areas.

[17][18] Another large tree measures 65 m high with a 6 m diameter trunk in the Blue Gum Forest in the Grose Valley near Blackheath.