Background: In the COMBI-AD trial, adjuvant D + T resulted in a significant relapse-free survival benefit (HR, 0.47 [95% CI, 0.39-0.58]; P< .001) vs pbo in pts with resected stage III BRAF V600E/K–mutant melanoma. At a median follow-up of 2.8 y, 37% of pts in the D + T arm and 57% in the pbo arm had relapsed. Resistance mechanisms are largely unknown in the adjuvant setting. We report mutational and immune gene expression profiling at relapse in COMBI-AD. Methods: COMBI-AD (NCT01682083) randomized pts with resected stage III BRAF V600E/K–mutant melanoma to receive D + T or pbo. DNA and RNA were extracted from the same relapse sample. Mutational landscape and immune gene expression signature were examined by sequencing 570 genes (mean depth ≈ 500×) and gene expression profiling using a customized NanoString panel. Plasma samples were subjected to ctDNA profiling (73 genes). Results: At relapse, 66 tissue samples (D + T, n = 24 [n = 4 relapsed on treatment]; pbo, n = 42) were collected (20 distant, 43 local/regional, 3 secondary primary melanoma). Paired baseline samples were available in most cases (n = 57). A BRAF V600E/K mutation was detected in all relapse samples except in 1 secondary primary melanoma. The genomic landscape was as expected for melanoma: the most common non–BRAF V600 genetic aberrations were CDKN2A (38%), CDKN2B (24%), PTEN (23%), TP53 (18%), and ARID2 (14%), and median tumor mutation burden (TMB) was ≈ 10 SNVs/Mb. No substantial differences in mutation frequency, TMB, or BRAF copy number were noted in relapse vs baseline samples or between treatment arms. Putative genetic resistance mechanisms in the MAPK (eg, MAP2K1, NRAS mutations) and non-MAPK (eg, PI3K pathway mutations) pathways were found in a small subset of relapse samples from pts treated with D + T. No significant differences in T-cell–specific/immune gene expression signatures were observed between arms. Conclusions: Although mutations in MAPK and non-MAPK pathways were found in a few relapse samples, no uniform adaptations were observed, suggesting that resistance to adjuvant D + T is caused by various genetic/epigenetic mechanisms. Clinical trial information: NCT01682083