Background: Over the past decade, advances in immunotherapy (IO) have dramatically improved overall survival (OS) for patients with metastatic melanoma. Immune checkpoint inhibitors and BRAF/MEK inhibitors also became standard of care for surgically resected stage III melanoma, and most recently pembrolizumab was approved for stage IIB/C patients. Although the phase III trials that led to their respective approvals met their primary endpoints for relapse-free survival, no benefit in OS has been observed. Given that a large proportion of early-stage melanoma patients are cured with surgery alone, the decision to proceed with adjuvant therapy should be weighed against the risk of potential life-altering toxicity. There is a high unmet need to develop biomarkers to accurately select patients who are most likely to benefit from adjuvant IO while sparing a group of low risk patients from unnecessary treatment. In this study, we evaluated the prognostic value of personalized, tumor-informed circulating tumor DNA (ctDNA) testing in patients with resectable stage II/III melanoma. Methods: In this real-world study, longitudinal plasma samples (n = 159) were analyzed in real-time from 45 stage IIA-IIID cutaneous melanoma patients treated at Rush University between 03/30/2021 and 01/12/2023. A personalized, tumor-informed ctDNA assay (Signatera bespoke, mPCR-NGS assay) was used for detection and quantification of ctDNA in plasma samples. Results: Personalized, tumor-informed ctDNA assays were successfully designed for 93% (42/45) of patients using tissue samples from either surgical resection (14/42, 33%) or biopsy (26/42, 67%; 16 shave, 10 excisional, 1 punch, 1 core). ctDNA was detectable at one or more time points in 36% (15/42) of patients. We observed a higher ctDNA-positivity rate in stage III patients (11/20, 55%) compared to stage II patients (4/22, 18%). Twelve of the 15 ctDNA-positive patients received adjuvant IO/treatment escalation after a positive ctDNA result; 6 patients had sustained ctDNA clearance, 2 had transient clearance, and 4 continue to receive adjuvant IO and follow-up ctDNA testing. Radiographic recurrence was observed in 7/42 patients, 6 of whom were ctDNA-positive. Ninety-six percent (26/27) of patients with persistent ctDNA-negativity remain clinically and radiographically progression-free. Conclusions: Our data suggest that molecular residual disease detection using a personalized, tumor-informed ctDNA assay is highly prognostic in melanoma patients undergoing curative resection. Sustained clearance of ctDNA was achieved with adjuvant therapy in a subset of this cohort. Prospective studies evaluating the utility of ctDNA to aid clinical decision-making regarding adjuvant treatment intensification or de-escalation are warranted.