This article uses the concept of quasi-vertical integration to attempt to explain the inter-firm relationships of parts sourcing that allowed Hyundai Motor to attain its high level of growth. This involved the integration of Hyundai Motor Group’s affiliated and non-affiliated parts suppliers across the boundaries of the firm. This study shows that the formation, systematization, and expansion of Hyundai Motor’s quasi-vertical integration did not develop under a consistent long-term plan, but rather evolved in response to environmental changes. Hyundai Motor’s quasi-vertically integrated supply chain formed in the 1980s with the creation of a captive subcontracting system backed by the government. As the modularization of production and quality management began to gain traction in the 2000s, the system became more refined and spread overseas as Hyundai Motors expanded its global production. The system has several theoretical implications. First, Hyundai Motor’s inter-firm relationships with vertical modularization go beyond conventional modularization that presupposes a horizontal relationship. Second, as an extension of MacDuffie (2013)’s concept of quasi-vertical integration that applies within the boundaries of Korea’s chaebol, this includes not only affiliates but also non-affiliates with no equity relationships across the boundaries of the firm. Finally, Hyundai Motor’s quasi-vertically integrated supply network differs from a Japanese keiretsu, a close network of mutual obligations between companies. Unlike in the Japanese automobile industry, where modular production in the 2000s weakened the keiretsu system, Hyundai Motor’s system became even more quasi-vertically consolidated through the advent of modular production.