Photoacoustic imaging (PA) has emerged as a novel and noninvasive imaging modality owing to its high spatial resolution and high soft tissue contrast. Herein, we loaded a near-infrared (NIR) fluorescence dye (CySCOOH) onto the surface of PEGylated graphene oxide (GO) via π-π stacking to increase the NIR absorbance of GO. The PA imaging proved that PEGylated GO-CysCOOH (GO-PEG-CysCOOH) significantly enhances the PA signal in the tumor site compared with free GO-PEG. We then utilized the strong optical absorbance of GO-PEG-CySCOOH in the NIR region for in vivo photothermal therapy, achieving efficient tumor ablation after intravenous injection of GO-PEG-CySCOOH and low-power laser irradiation on the tumor. Our results indicate that this graphene-based nanocomposite can be developed as a promising contrast agent for PA imaging and a thermal agent for imaging guided photothermal therapy.