In this study, an organic semiconducting pro‐nanostimulant (OSPS) with a near‐infrared (NIR) photoactivatable immunotherapeutic action for synergetic cancer therapy is presented. OSPS comprises a semiconducting polymer nanoparticle (SPN) core and an immunostimulant conjugated through a singlet oxygen (1O2) cleavable linkers. Upon NIR laser irradiation, OSPS generates both heat and 1O2 to exert combinational phototherapy not only to ablate tumors but also to produce tumor‐associated antigens. More importantly, NIR irradiation triggers the cleavage of 1O2‐cleavable linkers, triggering the remote release of the immunostimulants from OSPS to modulate the immunosuppressive tumor microenvironment. Thus, the released tumor‐associated antigens in conjunction with activated immunostimulants induce a synergistic antitumor immune response after OSPS‐mediated phototherapy, resulting in the inhibited growth of both primary/distant tumors and lung metastasis in a mouse xenograft model, which is not observed for sole phototherapy. An organic semiconducting pro‐nanostimulant was developed for synergetic cancer therapy. Only upon near‐infrared (NIR) irradiation is the pro‐nanostimulant activated to simultaneously exert phototherapy and checkpoint blockade immunotherapy, inhibiting the growth of primary/distant tumors and significantly suppressing the lung metastasis without causing any obvious in vivo toxicity.