Plants small, short and weak-stemmed, compact and sprawling in thin mats, green to pale yellow. Stems green, superficial cortex of thin-walled but not much enlarged or differentiated. Stem leaves elongate-triangular, 1.3-1.5 mm; often spreading; apex obtuse; hyaline cells mostly efibrillose and 1-septate in proximal half and lateral portions of leaves. Branches unranked, often blunt and with leaves moderately elongated at distal end. Branch fascicles with 2-3 spreading and 0-2 pendent branches. Branch stems green, with cortex enlarged with conspicuous retort cells. Branch leaves ovate to broadly ovate at branch base and becoming ovate-lanceolate at branch tip; 1.2-1.5 mm; undulate when dry, margins serrulate; hyaline cells of convex surface with 0-5 pores or pseudopores at cell apex, concave surface with faint round wall thinnings in cell angles, but may be absent, chlorophyllous cells trapezoidal in transverse section, exposed more broadly on convex surface. Sexual condition probably dioicous. Spores not seen.

Mats in seasonally wet depressions in coastal plain; low elevations; La., Miss., N.J.

Sporophytes of Sphagnum mississippiense are unknown. The combination of broad branch leaves and obtuse stem leaves will distinguish it from S. cuspidatum and S. viride. The much commoner and more wide-ranging S. trinitense, although also having serrulate branch leaves, has much narrower branch leaves that are more elongate at the branch tips, becoming quite lanceolate as compared with the ovate-lanceolate branch leaves that S. mississippiense exhibits at its branch tips.