Online retailers are among the world’s strongest brands. Yet, the strategic relevance of e-tailer brands remains elusive. This research therefore explores the strategic relevance of online retailer brands from a firm-perspective, utilizing theories-in-use techniques and grounded theory development. It draws on interviews with marketing executives from a diverse set of online retailers, collectively representing more than 2.4 bn USD in e-commerce revenue. The findings show a strong e-tailer brand to constitute a competitive advantage, proposing a substantial relationship between online retailer brand equity and e-commerce market share. This research is the first to unveil how this competitive advantage manifests in the marketplace, uncovering the driving mechanisms of the brand equity – market share link. Based on the present findings, corporate brand equity offers a plausible explanation for the astonishingly high market concentration in online retailing.