Two new species of Marasmius secton Sicci are described based on field work conducted in Phu Hin Rong Kla National Park, Thailand. Marasmius thailandicus is characterized by small, thin yellowish white basidiomes, ellipsoid basidiospores and dimorphic caulocystidia. Marasmius rongklaensis is distinguished by a conical, velutinous pileus, ellipsoid basidiospores, dimorphic cheilocytidia and pileipellis a hymeniderm with pileosetae. Comprehensive description along with illustrations, photographs, and a comparison with phenetically similar taxa are provided. The taxonomic position has been confirmed by phylogenetic analyses of the internal transcribed spacer (ITS1-5.8S-ITS2) sequences of the ribosomal DNA.