Betalain pigments, primarily produced by the order Caryophyllales, are categorized into betacyanins (red/purple) and betaxanthins (yellow/orange). While the biosynthetic pathways of these pigments are well-studied, the genes responsible for betaxanthin biosynthesis in quinoa were previously unknown. This study identified three candidate genes, CqCYP76AD5v1, CqCYP76AD5v2, and CqCYP76AD130, as quinoa orthologs of beet CYP76AD5 and CYP76AD6. Agroinfiltration experiments in Nicotiana benthamiana revealed that CqCYP76AD5v1 exhibited L-DOPA synthesis activity, whereas CqCYP76AD130 did not. To enable large-scale production of betaxanthins, we developed a tobacco BY-2 cell line expressing CqCYP76AD5v1 and CqDODA1-1, with vulgaxanthin I identified as the predominant product. Furthermore, the betaxanthin mixture extracted from this line inhibited amyloid-β (Aβ) aggregation, a key factor associated with Alzheimer's disease. These findings demonstrate the potential of betaxanthins derived from quinoa betaxanthin-biosynthesis genes for applications in health supplements and pharmaceuticals.