Context.—Corynebacterium kroppenstedtiimay cause granulomatous lobular mastitis (GLM). Yet, current understanding is limited regarding its overall clinical significance, cultivation practice, GLM pathogenesis, and antimicrobial treatment.Objective.—To report the isolation and significance of C kroppenstedtii, features of patients with GLM, pathologic findings and mechanism, bacteriologic workup, and optimal treatment.Design.—Analysis of the cases with C kroppenstedtii at The University of Texas MD Anderson Cancer Center from 2016 to March 2024 for mechanistic insights.Results.—During a period of 8 years, isolates of C kroppenstedtii were obtained from 10 women and 7 men. All of the women, with an average age of 34 years (range, 18–61 years), presented with chronic or subacute mastitis, and were subsequently diagnosed with GLM. The men, with an average age of 66 years, had neoplastic diagnoses with the bacterium being commensal in 6 cases. Thus, C kroppenstedtii shows a predilection to infect the female breast (P < .001). Predisposing risks for GLM included childbirth in 8 women and nipple inversion in 2 women. Histopathology revealed xanthogranulomatous inflammation and Gram-positive bacilli within fat droplets or extracellularly. From GLM aspirates or tissue, the liquid culture media and/or anaerobic incubation yielded 9 of 10 isolates. Up to 14 tested strains were susceptible to vancomycin, linezolid, rifampin, and gentamicin. Nine women received extensive antimicrobial therapy.Conclusions.—Corynebacterium kroppenstedtiiis a rare commensal, is mammophilic, and causes GLM. Liquid culture media and/or anaerobic incubation offer the best culture recovery. The inflamed fatty milieu of the breast forms xanthogranulomas. Several antibiotics likely render effective treatment, particularly lipophilic rifampin, which penetrates the bacteria-laden fat droplets.