An adjustable deck shelf assembly has repositionable components to enable the assembly to be secured to variably dimensioned and assembled deck railings. A shelf is secured to a hinged shelf support board with tight dimensional tolerances or with a removable locking shaft in a mechanical interference fit to permit removal and storage of the shelf and assembly. Hinged truss shelf supports secured to support legs can be deployed to support the shelf and retracted for storage. An adjustable deck shelf/seat assembly incorporates variable-height outriggers attached to truss shelf supports and a variable-height shelf support board to enable the assembly to be arranged as a shelf or as a seat. The shelf alternatively may be secured to the deck shelf assembly with slots and hooks.