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No clear evidence shows the association between particulate matter (PM) with an aerodynamic diameter < 10 µm (PM10) and asthma control. Therefore, the objective of this study was to determine the association between PM10 and asthma control. A retrospective observational study was conducted at the Airway Clinic, Chiang Mai University Hospital, Chiang Mai, Thailand, between January 2010 and April 2013. Various values of asthma control test (ACT) scores between high and low PM10 periods were analyzed. The association of an increased monthly average PM10 level and ACT score was analyzed using a time series analysis. There were a total of 1180 visits from 236 asthmatic patients. The monthly average ACT score was significantly lower in the high PM10 period compared with the low PM10 period. Every 10 µg/m3 increment of monthly average PM10 resulted in a significantly decreased ACT score at lag zero and one month, with an adjusted coefficient of –0.101 (95% CI; –0.165, –0.037), p-value = 0.002 and –0.079 (95% CI; –0.147, –0.012), p-value = 0.021, respectively. Monthly average PM10 significantly affected asthma control in asthmatic patients. During the air pollution period, the serial assessments of ACT should be measured for early detection of worsening asthma control.