The objective of this study was to investigate the seasonal patterns and trends of air pollutants in upper northern Thailand (UNT) from 2004 to 2018. The hourly air pollutant concentration data recorded from 6 monitoring stations in the UNT were obtained from the Pollution Control Department, Ministry of Natural Resources and Environment of Thailand. Cubic splines were used to assess seasonal patterns and trends of air pollutants. Linear regression was used to estimate the average increase in concentrations of air pollutants at each monitoring station. The results exhibited seasonal patterns for CO, NOX, NO2, O3, and PM10, in all stations while SO2 only in one station in Lampang and all stations in Chiangmai. The concentrations of these pollutants rose during August and September and reached peak levels between March and April. In the past 15 years, the levels of overall CO, O3, and SO2 in the UNT had significantly increased, on average of 0.032 ppm, 0.012 ppb, and 0.017 ppb, respectively. In contrast, NO2, NOX, and PM10 had significantly decreased on average of 0.012 ppb, 0.011 ppb, and 0.016 mg/m3, respectively. In conclusion, it should be of concern for such activities that related to air pollutants variation accordingly.