Gelatinomyces siamensis gen. sp. nov., incertae sedis within Leotiomycetes, the Siamese jelly-ball, is described. The fungus was collected from bamboo culms and branches in Nam Nao National Park, Phetchabun, Thailand. It presents as a ping-pong ball-sized and golf ball-like gelatinous ascostroma. The asci have numerous ascospores, are thick-walled, and arise on discoid apothecia which are aggregated and clustered to form the spherical gelatinous structures. An hyphomycete asexual morph is morphologically somewhat phialophora-like, and produces red pigments. On the basis of phylogenetic analysis based on rRNA, SSU, and LSU gene sequences, the lineage is closest to Collophora rubra. However, ITS sequences place the fungus on a well-separated branch from that fungus, and the morphological and ecological differences exclude it from Collophora.