Novel quaternary ammonium salt‐linked STING agonist ADC demonstrates high potency, stability, enhanced solubility, and reduced off‐target toxicity, potently activating the STING pathway in the tumor microenvironment through the synergistic action of the Fab and Fc regions of antibodies (activity switch‐on) while exhibiting ≈20 fold lower activity in normal immune cells, where activation relies solely on Fc (activity switch‐off). Here, STING1 is linked to neoplasm.