This study measured the efficacy of an online consumer knowledge in changing Millennials’ sustainable clothing purchase intention and offers managerial implications for similar websites. The responses of two groups to a 31-item survery were assessed using a two-groups structural equation modeling (SEM) comparison. ProjectJUST.com users were recruited using a banner appearing on the platform’s landing page (PJ group; n = 700), and a group of non-users was recruited using Amazon Mechanical Turk (MT group; n = 685). The group that was seeking sustainable clothing consumer knowledge already perceived the information as being personally relevant, which echoes prior findings (Dickson, 2001). This study’s results contribute that these information seekers already intended to purchase sustainable clothing when they found the website. Rather than fundamentally changing behavioral intention, these tools seem to offer shopping guides. To change clothes shopping priorities and increase website traffic from non-users, the content provided by online sustainable clothing consumer knowledge tools ought to target non-users’ perceived personal relevance. Acknowledging the contradiction between Millennials’ preference for sustainable products and disposable fashion could be one pressure point.