Based on light and scanning electron microscopical examinations, 2 new morphologically similar species of the subgenus Procamallanus (Spirocamallanus) (Nematoda: Camallanidae) possessing 2 caudal spikes are described from the intestine of marine perciform fishes from the Gulf of Thailand off the Thai coast, i.e., Procamallanus (S.) rigbyi n. sp. from Otolithes ruber (Bloch and Schneider) (Sciaenidae) and Procamallanus (S.) similis n. sp. from Sillago sihama (Forsskål) (Sillaginidae). The former species is mainly characterized by the presence of 13-14 spiral ridges in the buccal capsule, location of deirids at about mid-way between the buccal capsule and the nerve ring, and by the length of the right spicule (315-360 µm), whereas the latter is characterized by 10-12 spiral ridges, deirids situated at short distance posterior to the buccal capsule, and the right spicule 435-492 µm long. Because of their inadequate descriptions, most species of this subgenus reported from marine fishes of the Indo-Pacific region are considered species inquirendae. Spirocotyle Yasmin and Bilqees, 2007 is considered a junior synonym of Procamallanus Baylis, 1923. Since Procamallanus (S.) otolithi Ashraf, Farooq and Khanum, 1977 , P. (S.) otolithi (Gupta and Garg, 1986 ) and P. (S.) otolithi (Yasmin and Bilqees, 2007 ) are junior homonyms to P. (S.) otolithi Bilqees and Kazmi, 1974, they are renamed Procamallanus (S.) pakistanensis n. nom., Procamallanus (S.) problematicus n. nom., and Procamallanus (S.) incognitus n. nom., respectively (all species inquirendae). Procamallanus rigbyi and P. similis are the first nominal species of this genus reported from marine fishes off the coast of Thailand.