A new thecate, phototrophic, marine sand-dwelling dinoflagellate, Thecadinium mucosum Hoppenrath et Taylor sp. nov., is described from a culture isolated from Boundary Bay, British Columbia, Canada. It is illustrated with light microscopy as well as scanning- and transmission-electron microscopy, and its position in the phylogenetic tree of dinoflagellates is investigated using molecular methods. Cells are asymmetrical, oval, laterally flattened and strongly pigmented, with the plate formula P 3' 1a 6'' 7/8c 5s 6''' 2''''. Thecal plates are smooth with scattered pores, and there is a distinctive anterior intercalary plate that could be involved in mucus secretion. Thecadinium inclinatum Balech (= Sabulodinium inclinatum (Balech) Saunders et Dodge), a thecate, marine, sand-dwelling species that has been previously confused with what we now call T. mucosum, was also examined and illustrated through light- and scanning electron microscopy. New information on T. inclinatum is provided, including its plate formula: P 3' 6'' 7c ?s 5''' 1p 1''''; we consider T. inclinatum to be related to most other Thecadinium species and not to Sabulodinium. Molecular phylogenetic analyses based on the small subunit ribosomal gene of T. mucosum, T. kofoidii (the type species of the genus) and T. dragescoi weakly support earlier suspicions based on morphology that T. dragescoi is not a member of Thecadinium. Tabulational patterns of the species suggest a relationship to the genus Amphidiniopsis.