Rapid economic development and urbanization have caused severe pollution from PM2.5 in the Pearl River Delta (PRD) region of China. Characterizing the long-term PM2.5 trends is a basic requirement for evaluating the effectiveness of the implemented control measures and for guiding future policies. Traditional characterization has relied on fixed-site measurements, leading to incomplete spatial and temporal coverage. In this study, we took advantage of satellite-derived PM2.5 data at a resolution of 0.01° × 0.01° and assessed the long-term PM2.5 trends on a city scale in the PRD region and on a district scale in Hong Kong from 2001 to 2015. The results of our analysis showed that the PM2.5 concentration in the PRD region and Hong Kong worsened during the 10th Five-Year-Plan (FYP) period (2001–2005) and started to improve during the 11th FYP period (2006–2010). The improvement accelerated during the 12th FYP period (2011–2015). These trends were in line with the trends of air-pollutant emissions. In addition, the cities in the central area of the PRD region experienced higher PM2.5 concentration increase during the 10th FYP period and a greater reduction in the concentration afterward during the 11th FYP period than those in the non-central area of the PRD region. During the 12th FYP period, PM2.5 concentrations substantially lessened for most of the cities in the PRD region, particularly for those in the non-central area of the PRD region. For Hong Kong, the rates of increase and decrease in the PM2.5 concentration generally exhibited gradients from the northwest to the southeast, suggesting effects from the regional transport of air pollutants. A group of districts in the central urban area of Hong Kong also experienced a substantial decline in the PM2.5 concentration during the 11th and 12th FYP periods, suggesting the beneficial effects of local control efforts.