Lasianthus purpureocalyx Napiroon, Chamchumroon, Balslev & Chayamarit sp. nova (Rubiaceae) from the Khao Ngon Nak hill (Aaonang sub-district, Krabi province, Thailand) is described and illustrated. It is similar to L. subcalvus but differs from it in having narrowly lanceolate, 2–3.5 × 12–16 cm leaves, 6–7 corolla lobes, and globose fruits (versus leaves oblong-elliptic 5–7 × 16–23 cm, corolla 4-lobed, fruit fusiform). It also resembles L. purpureus in leaf morphology, but differs from it in having flowers in sessile cymes instead of fascicular pedicels. Its leaf morphology also bears resemblance to that of L. foxworthyanus, but differs from it in having a purple tubular calyx with 6–7 lobes and calyx limb that are shallowly curved or nearly entire (versus a pale green campanulate calyx with 4–5 lobes and 5 calyx limb lobes that are lanceolate).