Self-assembly of dyes has become a flexible strategy to modulate their photophysical properties. H-aggregates show great potential to increase heat generation, while the precise designing of H-aggregates as efficient photothermal agents is still challenging. Herein, a quinoline cyanine (QCy) is developed for constructing stable H-aggregated nanoparticles (NPs) to significantly enhance photostability and photothermal conversion efficiency (PCE). With symmetrical rigid planar quinoline structures, QCy has a small and symmetrical dihedral angle (11.9°), which ensures excellent molecular planarity. In aqueous solution, the planar QCy can form close π–π molecular stacking, and fast self-assemble into stable H-aggregates even at low concentrations (1 × 10⁻⁷ M). QCy H-aggregates are sphere-like NPs (QCy NPs) with an average diameter of 120 nm and exhibit high stability. H-aggregation of QCy significantly enhances PCE from 20.1% (non-H-aggregated QCy) to 63.8% (QCy NPs). In addition, the positive charge of quaternarized quinoline provides mitochondrial anchoring ability, which further enhances the photothermal effect. With high PCE and tumor accumulation, QCy NPs in low-doses have been successfully used in photoacoustic imaging-guided tumor photothermal therapy.