This study investigated PM2.5-bound polycyclic aromatic hydrocarbons (PAHs) in order to determine the seasonal changes in total benzo[a]pyrene equivalent (BaPeq) concentrations and to identify contamination sources by using a positive matrix factorization model, a conditional probability function, and characteristic ratios of PAHs in Hsinchu. The sampling period was from September 2014 to August 2015. PM2.5 samplers equipped with 47-mm quartz membrane filters were operated at a flow rate of 16.7 L min–1 for 48 h. The concentrations of 20 PAHs were determined through gas chromatography–mass spectrometry. The results revealed the PM2.5, total PAHs, and BaPeq mass concentrations in the four seasons ranged from 4.91 to 58.5 µg m–3, 0.21 to 8.08 ng m–3, and 0.03 to 0.78 ng m–3, respectively. The PM2.5, total PAHs, and BaPeq mass concentrations were in the order winter > autumn > spring > summer and exhibited significant seasonal variations. The carcinogenic potency of PAHs in winter was approximately 6.21 times higher than that in summer. The major BaPeq contributors were BaP, BbF, INP, and DBA. BaP accounted for 49.0% of BaPeq concentrations in PM2.5 in all four seasons. The annual average lifetime excess cancer risk of PM2.5-bound PAHs (1.60 × 10–5) was higher than that specified in the United States Environmental Protection Agency guidelines (10–6). The two major sources were stationary emission sources and unburned petroleum and traffic emissions, which together accounted for 90.3% of PM2.5-bound PAHs.