Rhododendron caeruleum H. Léveillé; R. eriandrum H. Léveillé ex Hutchinson; R. hesperium I. B. Balfour & Forrest; R. racemosum Franchet var. rigidum (Franchet) Rehnelt; R. rarosquameum I. B. Balfour; R. sycnanthum I. B. Balfour & W. W. Smith.

Shrubs, rarely small trees, 1–2(–10) m tall; old branches glabrous; young shoots with scales absent or sparse, sometimes also pubescent. Petiole 2–12 mm, scales sparse or absent, otherwise glabrous or rarely pubescent; leaf blade elliptic, oblong elliptic, oblong-lanceolate or oblanceolate, 2.5–6.8 × 1–3.2 cm; base narrow or rounded; apex obtuse, acute, or rounded, apiculate; abaxial surface gray green, scales (2–)4–8 × their own diameter apart, or contiguous, unequal; adaxial surface brownish green, sparsely scaly or absent, sometimes also pubescent along midrib. Inflorescence terminal or subterminal, shortly racemose, 2–6-flowered; rachis 2–4 mm. Pedicel 0.5–2 cm, not scaly or sparsely scaly and pubescent; calyx rimlike or 5-lobed, 0.5–1 mm, scales present or absent; corolla broadly funnelform-campanulate, pale purplish to white or deep reddish purple, with greenish brown or purple spots within, 1.8–3 cm, outer surface not scaly, rarely scaly; stamens unequal, long ones longer than corolla; filaments densely pubescent at base; ovary 5-locular, densely scaly, rarely pubescent on top; style longer than corolla, glabrous. Capsule cylindric, 8–10 mm.

Rhododendron bodinieri Franchet (J. Bot. (Morot) 12: 257. 1898), described from S Yunnan, is intermediate between, and may be a hybrid between, R. rigidum and R. siderophyllum (see Cullen, Notes Roy. Bot. Gard. Edinburgh 39: 69. 1980).

Forest margins, thickets; 2000–3400 m. Sichuan, Yunnan.