Photothermal therapy （PTT）， as a non-invasive treatment strategy， utilizes photothermal agents （PTAs） to generate localized hyperthermia under light excitation for specific killing of cancer cells. Due to its precise spatiotemporal controllability and remarkable therapeutic efficacy， PTT has emerged as a research focus in the field of precision oncology in recent years. Compared to the conventional near-infrared window （NIR-I， 700~900 nm）， the second near-infrared window （NIR-Ⅱ， 1000~1700 nm） offers superior tissue penetration depth （> 10 mm）， spatial resolution， and signal-to-noise ratio （SNR）， owing to reduced tissue absorption and scattering， thereby creating new opportunities for the treatment of deep-seated tumors. However， current research on NIR-Ⅱ-responsive PTAs has predominantly focused on inorganic nanomaterials （such as metallic oxide and quantum dots） and polymeric macromolecular systems. The potential biotoxicity， poor metabolic clearance， and complex synthesis processes of these nanomaterials limit their clinical translation. Herein， we review recent advances in NIR-Ⅱ organic small-molecule PTAs by systematically elucidating the design principles of organic semiconducting small molecules with different backbones and thoroughly analyzing the corresponding structure-activity relationships. Furthermore， this review evaluates their performance in deep-tumor NIR-Ⅱ PTT and multimodal combination therapy. Finally， in light of the challenges facing organic semiconducting small-molecule PTAs， such as unclear in vivo pharmacokinetics and unverified long-term biosafety， we propose future research directions. These include molecular engineering optimization， biomimetic functionalization， and the construction of smart theranostic nanoplatforms. This contribution will provide a theoretical foundation for advancing the clinical translation of NIR-Ⅱ organic photothermal agents.