Rhizome erect, scaly; rhizome scales very large, dark brown, of two types, long filiform, extremely fragile, thus almost all broken, larger scales with broad base, more than 18.38 x 1.95 (base) mm, smaller scales linear, more than 15.3 x 0.74 (base) mm. Fronds monomorphic, tufted. Stipes 40–47(–108) × 0.7 cm, light green, at base dark brown, canaliculate, base with scales similar to those from rhizome. Lamina 2–pinnate–pinnatifid to nearly 3–pinnate, triangular–ovate, 59–69(–105) × 47.5–64 cm, green, shiny on upper surface; pinnae in ca. 15–18 pairs, alternate, arranged at an angle of ca. 45°, (15.3–)23.7–34 × 14.5–17 cm; pinnules in 12–14(–19) pairs in each pinna, opposite, (4.5–)5.1–6.6(–9.2) × 1.6–2.3(–4.5) cm, oblong–lanceolate; pinnules 3 rd order in 6–10(–12) pairs tapering into the caudate apex, opposite, 0.9–1.1 × 0.4–0.6 cm, decurrent, deeply incised; rachis greenish–brown to middle– brown, with few scales, pinnae rachis light–brown to greenish, base often darker, with few scales, sometimes minutely hairy; racheolule mostly winged. Sori in basic pinnule lobes up to 6 sori, in all other sori usually in two rows along midrib, 6–8(11), elongate, 1.98–2.95 × 0.71–0.98 mm; indusium elongate, pale, glabrous, same size as sori attached to one side of sorus (asplenioid).

The new species resembles Diplazium nemorale which has shallowly to deeply cut pinnules (deeply incised to almost pinnate in D. cyamudongoense), a lamina that does not appear tripinnate and is almost undivided towards pinna apex and on upper pinnae (all pinnae up to the terminal pinna are pinnatifid in D. cyamudongoense), sori 1–6 per pinnule lobe and 3–4(5) mm long (6–8(11) sori per pinnule lobe and 1.98–2.95 mm long in D. cyamudongoense), basal stipe scales not exceeding 9 mm in length (more than 18 mm long in D. cyamudongoense).

cyamudongoense: from Cyamudongo Forest, Rwanda. Until now, the new species is only known from two localities in this high biodiversity area.

Montane forest along streams, altitude 1877–1941 m.

See African distribution.

Rwanda.

Terrestrial.