Fimbristylis fusiformis, an unusual new species of Cyperaceae from Thailand, is described and illustrated. This taxon has a single terminal spikelet per culm with a semi-distichous glume arrangement, bisexual flowers that lack perianth parts, and pistil with persistent style whose base is slightly swollen and trigonous nutlets with pubescent ribs. Phylogenetic reconstruction using ITS sequence data places this taxon in Abildgaardieae and sister to the rest of Fimbristylis. The species has a conservation status of Least Concern (LC).

A new species of Phyllagathis (Melastomataceae), P. nanakorniana, from Thailand is described and illustrated from recently collected material. The morphological characteristics are discussed in view of a wider generic concept that allows the inclusion of P. nanakorniana in Phyllagathis. A key to the Thai species is provided.

New combinations for five taxa in the Hedyotis-Oldenlandia complex (Rubiaceae) in Thailand are proposed, namely Debia rosettifolia, Dimetia capitellata var. pubescens, D. prainiana, D. ridleyana and Scleromitrion kerrii. Six relevant taxa are lectotypified.

Hedyotis puffii is described and illustrated based on material from Thailand. The new species differs from H. tavoyensis by its obtuse angled stems, usually with (0­­ ̶ )1 ̶ 2 glands on the calyx margin and the sinus of the calyx that has no interposed subulate teeth. The conservation status of this new taxon is vulnerable (VU D2). In addition the new combination Hedyotis laotica is made and a lectotype is designated.

Phanera larseniana, a new species from north–eastern Thailand, is described and illustrated.  It most closely resembles P. rubro-villosa but differs in the length of the floral parts, and in the number and position of the staminodes. The species is known only from a single locality in north-eastern Thailand. An illustration and photos of the new species are provided.

Two species of Gaertnera in Thailand, viz. G. junghuhniana and G. sralensis, are enumerated and described. A lectotype is designated for G. taiensis, a synonym of G. junghuhniana. A key to species and data on distribution, habit and phenology are also provided.