Abstract: The air filtration system for a box fan is a frame that couples to a box fan and holds an air filter adjacent to the box fan such that air drawn into the box fan must pass through the air filter. The front of the frame includes a fan mount that retains the fan in the frame using a press-fit. The rear of the frame comprises a filter mount that holds an air filter in the path of air flowing through the fan. The filter mount includes a top door that retains the filter and allow access to install and remove filters. In some embodiments, the air filtration system for a box fan may include a second fan mount to hold a second air filter.