People with diabetes mellitus (DM) are at increased risk for adverse health events and complications throughout their lifetime. Whether DM significantly affects collegiate athletes' concussion baseline testing performance remains unclear. To (1) describe the prevalence of DM and associated comorbidities and (2) compare concussion baseline testing performance between student-athletes with DM and student-athletes without DM (NoDM). Retrospective, cross-sectional study. University. Using the Concussion, Assessment, Research and Education (CARE) Consortium research database, we matched athletes with self-reported DM (N = 229) by institution, sex, age, sport, position, testing year, and concussion history to athletes with NoDM (N = 229; total sample mean age = 19.6 ± 1.4 years, women = 42%). Descriptive statistics and χ2 tests of independence with subsequent odds ratios were calculated. Independent-samples t tests compared baseline symptoms, neurocognitive testing, and balance performance between athletes with DM and athletes with NoDM. Effect sizes were determined for significant group differences. At baseline, athletes with DM had higher rates of self-reported pre-existing balance disorders, sleep disorders, seizure disorders, motion sickness, learning disorders, vision and hearing problems, psychiatric disorders, depression, bipolar disorder, nonmigraine headaches, and meningitis than athletes with NoDM (P values < .05). We found balance differences between groups (P = .032, Cohen d = 0.17) such that, on average, athletes with DM had 1 additional error on the Balance Error Scoring System (DM = 13.4 ± 6.5; NoDM = 12.1 ± 5.9). No other comparisons yielded significant results. Although athletes with DM had high rates of self-reported balance disorders, sleep disorders, seizures, and meningitis, their baseline neurocognitive testing results were largely identical to those of athletes with NoDM. Our findings suggested that nonclinically meaningful differences were present in concussion baseline balance testing but no significant differences were noted in cognitive testing; however, the effect of DM on concussion recovery remains unknown.