BACKGROUND: Relationships between air quality and health are well-described, but little information is available about the joint associations between particulate air pollution, ambient temperature, and respiratory morbidity. OBJECTIVES: We evaluated associations between concentrations of particulate matter /= 7.07 mug/m3 was associated with an estimated 4-5% higher asthma symptom prevalence. In the range of 4.00-7.06 mug/m3 of PM2.5, each 1-mug/m3 increase was associated with a 3.4% [95% confidence interval (CI): 1.1, 5.7] increase in symptom prevalence; across categories of temperature from 1.1 to 80.5 degrees F, each 1-mug/m3 increase was associated with increased symptom prevalence (1.1-44.4 degrees F: 7.9%; 44.5-58.6 degrees F: 6.9%; 58.7-70.1 degrees F: 2.9%; 70.2-80.5 degrees F: 7.3%). CONCLUSIONS: These results suggest that each unit increase in PM2.5 may be associated with an increase in the prevalence of asthma symptoms, even at levels as low as 4.00-7.06 mug/m3. Citation: Mirabelli MC, Vaidyanathan A, Flanders WD, Qin X, Garbe P. 2016. Outdoor PM2.5, ambient air temperature, and asthma symptoms in the past 14 days among adults with active asthma. Environ Health Perspect 124:1882-1890; http://dx.doi.org/10.1289/EHP92.