As they decrease in size, microplastics pose increasing environmental and health risks. Previous work showed that bioretention cells, a type of low impact development (LID), are effective at removing microplastics greater than approximately 100 μm from urban stormwater runoff. This two-year field study investigates whether bioretention cells provide similar benefits by removing microplastics as small as 25 μm in size from urban stormwater. The use of automated μFTIR mapping allowed for the analysis of smaller microplastics, less than 100 μm, which, until recently, have rarely been analyzed in stormwater due to the difficulty of their identification. A 71% concentration decrease was observed in the bioretention cell. In this 25–100 μm size range, the median microplastic concentrations were 227 microplastics/L in the stormwater (i.e., the bioretention inlet) and 66.5 microplastics/L at the outlet. The most prevalent synthetic polymers were polypropylene and polyethylene. Rubber and fibers were not analyzed due to method limitations. No correlations between hydrologic characteristics and microplastic quantities were observed, highlighting that other factors are likely involved in the fate and transport of microplastics in stormwater, like weather-induced particle fragmentation. These results demonstrate that this filtration-based LID system continues to provide effective microplastic removal down to 25 μm.