Plants fairly slender to moderate-sized, green; forming low dense hummocks. Stems yellowish green; 3 layers of superficial cortical cells. Stem leaves generally longer than branch leaves, 1.1-1.7 mm, lingulate to lingulate-spathulate, hyaline cells mostly non-septate. Branches terete. Branch fascicles of 2 spreading and 1-2 hanging branches. Branch stems with 1-2 layers of cortical cells. Branch leaves 1-1.4 mm, broadly ovate, with a narrow involute tip; hyaline cells only slightly bulging on either surface, in proximal half of leaf aporose on convex surface and with large faint pores on concave surface; internal commissural walls distinctly papillose; chlorophyllous cells elliptical to elliptical-triangular in transverse section, enclosed on both surfaces with the widest part in the leaf middle. Sexual condition dioicous. Spores ca. 31 µm, ornamented by small somewhat amalgamated granulae.

Sporophytes abundant, capsules mature August. Ecology poorly known but probably quite minerotrophic; low elevations; Alaska.

Sphagnum mirum has only been recently discovered and so far is known only from its type locality, where it was growing in a fen mixed with S. teres.