Photoacoustic tomography (PAT) is a newly emerging noninvasive imaging modality that could be further enhanced using near-infrared (NIR) absorbing materials as contrast agents. To date, the most extensively studied photoacoustic imaging agents are inorganic nanomaterials because organic materials with NIR-absorption capabilities are limited. In this study, a NIR-absorbing nanocomplex composed of a squaraine dye (SQ) and albumin was prepared based on the aggregation-induced NIR absorption of SQ. Through aggregation, the absorption spectrum of SQ was widened from the visible light region to the NIR region, which facilitated photoacoustic signal generation in the tissue-transparent NIR optical window (700 nm to 900 nm). Blood analysis and histology measurements revealed that the nanocomplex can be used for PAT applications in vivo without obvious toxicity to living mice.