Stemphylium blight (SB) (Stemphylium botryosum) is a major disease of lentil (Lens culinaris Medik.) in many global production regions. In Canada, SB typically occurs after canopy closure in the second half of the growing season when weather conditions become warm and humid. Experiments were carried out to assess the impact of SB disease on milling qualities of lentil. In 2014, milling qualities of red lentils were compared between seed obtained from an uninfected site and those produced at a naturally SB‐infected site. In 2016 and 2017, controlled field experiments were conducted by artificially inoculating the plots with SB conidia at the early flowering stage. Seed yield in the infected plots was reduced by 52, 22, and 30% compared with control plots in 2014, 2016, and 2017, respectively. The percentages of stained and deformed seeds were increased in infected plots in 2016 and 2017. Seeds infected with SB have reduced milling qualities. Dehulling efficiency was reduced by 25.3, 23.0, and 15.9%, and milling recovery was reduced by 23.7, 25.4, and 15.1% compared with the uninfected control plots, respectively, in 2014, 2016, and 2017. Stemphylium blight had minimal impact on football lentil recovery. These results suggest that SB is potentially the leading cause of the reported poor milling quality of Canadian red lentils that reduces the economic value in years when the growing season is favorable for development of SB. These results have major implications for agronomy, pathology, and breeding strategies of the lentil crop in temperate production regions.