Herein we report the synthesis and activity of an enzyme‐directed immunostimulant with immune cell activation mediated by β‐galactosidase, either exogenously added, or on B16 melanoma cells. Covalent attachment of a β‐galactopyranoside to an imidazoquinoline immunostimulant at a position critical for activity resulted in a pro‐immunostimulant that could be selectively converted by β‐galactosidase into an active immunostimulant. The pro‐immunostimulant exhibited β‐galactosidase‐directed immune cell activation as measured by NF‐κB transcription in RAW‐Blue macrophages or cytokine production (TNF, IL‐6, IL‐12) in JAWSII monocytes. Conversion of the pro‐immunostimulant into an active immunostimulant was also found to occur using β‐galactosidase‐enriched B16 melanoma cells. In co‐culture experiments with either immune cell line, β‐galactosidase‐enriched B16 cells effected activation of bystander immune cells.