A new polyketide glycoside, cladionol A (1), was isolated from the cultured broth of a fungus Gliocladium sp., which was separated from sea grass Syringodium isoetifolium, and the structure was elucidated by spectroscopic data. The relative stereochemistry of C-2-C-3 was assigned by mainly J-based configuration analysis, while those of two sugar units were elucidated to be beta-mannopyranoside and arabitol on the basis of NOESY data and/or 1H-1H couplings. Furthermore, the absolute configuration of the mannose moiety was determined as the D-form on the basis of chiral HPLC analysis of a benzoyl derivative of the acid hydrolysate of 1. Cladionol A (1) exhibited modest cytotoxicity.