A checklist of the megalopteran specimens housed in the Natural History Museum of the National Science Museum, Thailand (THNHM) is presented. A total of 583 specimens of 21 species, eight genera in two families (Corydalidae and Sialidae) including two holotypes and 26 paratypes are deposited in THNHM. Six species of Megaloptera are new records for Thailand: Acanthacorydalis asiatica (Wood-Mason, 1884); Ctenochauliodes yangi Liu and Yang, 2006; Neochauliodes meridionalis van der Weele, 1909; Neochauliodes tonkinensis (van der Weele, 1907); Neoneuromus ignobilis Navás, 1932; and Neoneuromus indistinctus Liu, Hayashi and Yang, 2018. Most specimens of the order were collected from Northern and Northeastern Thailand. The high-resolution images of the named species are provided.