Nonadherence and nonpersistence to antidepressants in major depressive disorder (MDD) are common and associated with poor clinical and functional outcomes and increased health care resource utilization (HCRU) and costs. However, contemporary real-world evidence on the economic effect of antidepressant nonadherence and nonpersistence is limited. To assess the effect of nonadherence and nonpersistence to antidepressants on HCRU and costs in adult patients with MDD enrolled in U.S. commercial and Medicare supplemental insurance plans. This was a retrospective new-user cohort study using administrative claims data from the IBM MarketScan Commercial and Medicare Supplemental databases from January 1, 2010, to December 31, 2018. We identified adult patients with MDD aged ≥ 18 years who initiated antidepressant therapy for a new MDD episode between January 1, 2011, and December 31, 2017. Twelve-month total all-cause HCRU and costs (2019 U.S. dollars) were characterized for patients who were adherent/nonadherent and persistent/nonpersistent to antidepressants at 6 months. Adherence was defined as having proportion of days covered (PDC) ≥ 80%, and persistence was defined as having continuous antidepressant therapy without a ≥ 30-day gap. Multivariable negative binomial regression and 2-part models adjusted for baseline characteristics were used to estimate incidence rate ratios (IRRs) for HCRU and incremental costs of nonadherence and nonpersistence, respectively. A total of 224,645 patients with MDD (commercial: n = 209,422; Medicare supplemental: n = 15,223) met all study inclusion criteria. Approximately half of patients were nonadherent (commercial: 48%; Medicare supplemental: 50%) or nonpersistent (commercial: 49%; Medicare supplemental: 52%) to antidepressants at 6 months. After controlling for baseline characteristics, nonadherent patients experienced significantly more inpatient hospitalizations (commercial, adjusted IRR [95% CI]: 1.34 [1.29 to 1.39]; Medicare supplemental: 1.19 [1.12 to 1.28]) and emergency room (ER) visits (commercial, adjusted IRR [95% CI]: 1.43 [1.40 to 1.45]; Medicare supplemental: 1.28 [1.21 to 1.36]) compared with adherent patients. Similar results were observed in nonpersistent patients. Adjusted mean differences revealed that nonadherent and nonpersistent patients accumulated significantly higher medical costs (commercial: $568 [95% CI: $354 to $764] and $491 [$284 to $703]; Medicare supplemental: $1,621 [$314 to $2,774] and $1,764 [$451 to $2,925]), inpatient costs (commercial: $650 [$490 to $801] and $564 [$417 to $716]; Medicare supplemental: $1,546 [$705 to $2,308] and $1,567 [$778 to $2,331]), and ER costs (commercial: $130 [$115 to $143] and $129 [$115 to $142]; Medicare supplemental: $82 [$23 to $150] and $80 [$18 to $150]), and incurred significantly lower pharmacy costs (commercial: -$561 [-$601 to -$521] and -$576 [-$616 to -$540]; Medicare supplemental: -$510 [-$747 to -$227] and -$596 [-$830 to -$325]) compared with adherent and persistent patients, respectively. This study found more hospitalizations and ER use and higher total medical costs among patients who were nonadherent and nonpersistent to antidepressants at 6 months. Strategies that promote better adherence and persistence may lower HCRU and medical costs in patients with MDD. This study was sponsored by Allergan, which was involved in the study design; data collection, analysis, and interpretation of data; and decision to present these results. Ta was supported by a training grant provided to the University of Washington by Allergan at the time this study was conducted. Tung and Gillard are employees of Allergan. Oliveri is an employee of Genesis Research. Sullivan and Devine have no financial disclosures. This study was presented as a poster at AMCP 2020 (Virtual Meeting), April 21-24, 2020.