Phototheranostics represents a highly promising paradigm for cancer therapy, although selecting an appropriate optical imager for clinical use remains challenging. We evaluated the phototheranostic nanoparticle-related properties of liposomally formulated phospholipid-conjugated indocyanine green, denoted as LP-iDOPE, for cancer diagnosis and treatment using photobiomodulation with a near-infrared (NIR)-light emitting diode (LED) light irradiator. Using in vivo NIR fluorescence imaging, we demonstrated that LP-iDOPE was selectively delivered to tumor sites with high accumulation and a long half-life. Following NIR-LED light irradiation of the tumor region of LP-iDOPE accumulation, effector CD8+ T cells were activated at the secondary lymphoid organs, migrated, and subsequently released cytokines including interferon-γ and tumor necrosis factor-α, resulting in effective tumor regression. Our cancer immunotherapy strategy using LP-iDOPE as a phototheranostic nanoparticle and an NIR-LED light irradiator as a photobiomodulation device represents a promising approach to noninvasive cancer therapy.