<p>Dairy farming is one of the crucial anthropogenic sources of airborne bacteria and aerosols for polluting and degrading environmental quality. An investigation was conducted to determine the indoor aerosols, microclimatic parameters, and airborne bacteria in an open type of dairy milking barn in Tochigi, Japan, from April 2017 to May 2018. The highest temperature (24.45ºC) and highest relative humidity (RH) (80.61%) were observed during the summer and the lowest temperature (4.27ºC) and lowest RH (64.0%) were observed during the winter and spring, respectively in the open type of dairy milking barn. A significant difference in airflow was not found during different seasons in the open type of dairy barn. All sizes of the aerosols (0.3–10.0&#xa0;μm) and all types of airborne bacteria (airborne aerobic bacteria, <i>Escherichia coli</i> (<i>E. coli</i>), and <i>Staphylococcus aureus</i> (<i>S. aureus</i>)) were greater during the summer and lower during the winter in the open type of dairy barn. A total of 13 (thirteen) relationships were observed among microclimatic factors, airborne bacteria, and aerosols in the dairy barn during the study period. A more significant negative relationship (<i>R</i> = 0.91, <i>p</i> = 0.01) was observed between the 1.0–2.0&#xa0;μm aerosols and airborne <i>E. coli</i> during the summer and a more significant positive relationship (<i>R</i> = 0.88, <i>p</i> = 0.02) was observed between airborne <i>E. coli</i> and temperature during the summer in the barn. These findings will be crucial in limiting emission of aerosols and airborne bacteria from various types of dairy facilities.</p>