Docosahexaenoic acid (DHA) is an essential ω-3 polyunsaturated fatty acid (PUFA) with high nutritional and pharmaceutical value. The marine protist Aurantiochytrium is a promising industrial DHA producer; however, its DHA biosynthesis via the PUFA synthase pathway co-produces ω-6 docosapentaenoic acid (DPA), limiting DHA purity. Here, we introduced an ω-3 desaturase from Phytophthora infestans (Pin-O3D) into Aurantiochytrium sp. SD116. Functional validation in an Escherichia coli system co-expressing the native PUFA synthase confirmed that Pin-O3D converts DPA to DHA, shifting the DHA/DPA ratio from 1:1 to 2:1. Pin-O3D was then integrated into the fatty acid synthase (FAS) locus, simultaneously attenuating FAS activity and enabling heterologous gene expression. The engineered strain ΔFAS-Pin-O3D exhibited significantly (p < 0.0001 in t-test) increased DHA content (55.2% of total fatty acids) and DHA/DPA ratio (5.91) in shake flasks, with no negative impact on biomass or lipid accumulation. Fed-batch fermentation confirmed the scalability of this strategy, achieving a >20% increase in DHA/DPA ratio. This study demonstrates that combining heterologous ω-3 desaturase expression with FAS attenuation is an effective approach for optimizing PUFA profiles in Aurantiochytrium.