ABSTRACT COA6/C1ORF31 is involved in cytochrome c oxidase (complex IV) biogenesis. We present a new pathogenic COA6 variant detected in a patient with neonatal hypertrophic cardiomyopathy and isolated complex IV deficiency. For the first time, clinical details about a COA6‐deficient patient are given and patient fibroblasts are functionally characterized: COA6 protein is undetectable and steady‐state levels of complex IV and several of its subunits are reduced. The monomeric COX1 assembly intermediate accumulates. Using pulse‐chase experiments, we demonstrate an increased turnover of mitochondrial encoded complex IV subunits. Although monomeric complex IV is decreased in patient fibroblasts, the CI/CIII2/CIVn‐supercomplexes remain unaffected. Copper supplementation shows a partial rescue of complex IV deficiency in patient fibroblasts. We conclude that COA6 is required for complex IV subunit stability. Furthermore, the proposed role in the copper delivery pathway to complex IV subunits is substantiated and a therapeutic lead for COA6‐deficient patients is provided. Cytochrome c oxidase (complex IV) levels are strongly reduced in cultured fibroblasts of a patient with a homozygous pathogenic variant in COA6 and severe hypertrophic neonatal cardiomyopathy. Copper supplementation partially recues complex IV deficiency in these cells providing a potential therapeutic lead for future patients.