•Increased severity status was associated with incremental burden in patients with TRD.•Inpatient admissions and outpatient visits were key drivers of incremental costs.•Pharmacotherapy use was similar across the severity cohorts. Among patients with major depressive disorder (MDD), those with treatment-resistant depression (TRD) have a higher economic burden. However, the healthcare resource utilization (HRU) and costs may vary by severity status in TRD patients. This study quantified the incremental economic burden of severity status in TRD patients. In a US database of privately insured employees and dependents (07/01/2009–03/31/2015), a claims-based algorithm identified adult TRD patients who were stratified into mild, moderate, and severe cohorts based on the information in the last observed MDD ICD-9-CM code. HRU and costs of moderate and severe cohorts were compared to those of the mild cohort during the 2-year follow-up after the first antidepressant claim. Among 6411 TRD patients, 455 (7.1%) were identified as mild, 2153 (33.6%) as moderate, and 1455 (22.7%) as severe. Moderate and severe patients compared to mild had 45% and 150% more inpatient admissions, 65% and 164% more inpatient days, 18% and 54% more emergency department visits and 8% and 10% more outpatient visits per-patient-per-year (PPPY), respectively (all-cause; all p < 0.05). Mean all-cause direct total healthcare costs were $12,123, $16,885, and $18,911 PPPY in mild, moderate, and severe patients, respectively. The all-cause total healthcare cost differences adjusted for baseline characteristics amounted to $3455 in moderate and $5150 in severe versus mild patients, respectively (PPPY; all p < 0.05). Not all TRD patients had a severity specifier; the severity specifier was not cross-validated against a depression scale. Increased severity status is associated with incremental economic burden in TRD patients.