Sustainability is currently regarded as an imperative business goal by multiple stakeholders, comprising investors, customers, and policymakers (Nidumolu, Prahalad, & Rangaswami, 2009; Sheth, Sethia, & Srinivas, 2011). In particular, how effectively the fashion industry deals with the challenges of sustainability will define its success for eras to come. This study focuses on how social power, parasocial interaction, and social capital work for purchase intention of sustainable fashion products in the fashion YouTube context. Specifically, the study investigates the effects of social power on parasocial interaction, the effects of parasocial interaction on social capital, and the effects of social capital on purchase intention for sustainable fashion products and the implications for sustainable fashion marketing and management. Theoretical Framework This study defines social power as types of power that can be employed to exert influence on others. The five social power bases (French & Raven, 1959) are discussed in terms of perceived influence: Expert power refers to someone who is perceived to be an expert, to have expert knowledge, or to possess special information. Legitimate power relates to someone who is perceived to have a legitimate right to impose behavioral requirements. Referent power is associated with someone who is personally identified. Reward power refers to someone who is perceived to have ability and coercive power to someone who is perceived to have the capability to confer punishment. Parasocial interaction concerns the relationship between media personalities and media users (Frederick, Lim, Clavio, & Walsh, 2012; Horton & Wohl, 1956; Jin & Park, 2009). Parasocial interaction can be defined as “immediate, personal, and reciprocal, but these qualities are illusory and presumably not shared by the speaker” (Horton & Strauss, 1957, p. 580; Jin & Park, 2009). Parasocial interaction theory focuses on the way audiences interact, relate to, and develop relationships with a celebrity (Jin & Park, 2009; Lee & Watkins, 2016). Audiences create a strong bond and intimacy with a celebrity while viewing media channels such as TV programs and social interactive media where audiences feel closer to the celebrity (Kassing & Sanderson, 2009; Lee & Watkins, 2016). Social capital refers to “the aggregate of the actual or potential resources linked to possession of a durable network of more or less institutionalized relationships” (Bourdieu, 1985, p. 248). Social capital involves the relationship between providing access to resources possessed by the associates and the nature and amount of those resources (Portes, 1998). Social capital can be clarified as an intangible force that helps to bind society together by transforming self-seeking individuals into members of a community with shared interests, shared assumptions about social relations, and a sense of the common good (Etzioni, 1996). Sustainability refers to three dimensions: economic, environmental, and social (Sheth, Sethia, & Srinivas, 2011). Sustainability transforms into a triple bottom line responsibility, with the inference that assessment of business outcomes should be based not only on economic performance, but also on the environmental and social impact. Environmental and social demands from various stakeholders contribute to the pressure for businesses to reflect sustainability. Thus, sustainable marketing practices are defined from economic, environmental, and social perspectives. In this study, effective sustainability measurements involve purchase intention for sustainable products especially emphasizing environmental and social performance. Focused on the effects of social power on parasocial interaction and the effects of parasocial interaction on social capital and purchase intention for sustainable products, this study tests the following hypotheses: H1. Social power (expert, referent, legitimate, and reward) positively influences parasocial interaction. H2. Parasocial interaction positively influences social capital (bonding and bridging). H3. Social capital positively influences purchase intention for sustainable fashion products (environmentally and socially sustainable fashion products). Methods This study used a survey to investigate key questions about the associations among social power, parasocial interaction, social capital, and purchase intention for sustainable fashion products. A total of 230 fashion YouTube users recruited from South Korea participated in the survey. Of the 230 participants, 40 were men (17.4%) and 190 were women (82.6%), with ages ranging from 20 to 39 (mean = 29.43 years). The social power of the fashion YouTuber (e.g., vlogger) was measured through an existing social power scale including expert, referent, legitimate, and reward measures that elicited user responses to 14 items (Goodrich & Mangleburg, 2010). Parasocial interaction was measured on the basis of user responses to six items on an existing 5- point scale that assessed parasocial interaction (Jin & Park, 2009). This study measured social capital on a 5-point Likert scale (1 = strongly disagree; 5 = strongly agree), which was adapted from an existing Internet social capital scale (Williams, 2006). Purchase intention was measured using three 7-point semantic differential scales (likely/unlikely, probable/improbable, possible/impossible; MacKenzie, Lutz, & Belch, 1986) after informing participants that they might be purchasing environmentally and socially sustainable products. Results The overall goodness-of-fit for this measurement model was acceptable (Chi-square 1236.138, df = 680, p <0.001, chi/df=1.818, TLI = 0.900, CFI = 0.913, RMSEA = 0.060). The reliability coefficients of all 14 social power measures including expert, referent, legitimate, and reward were 0.871, 0.782, 0.657, and 0.865, respectively. The reliability coefficient of all six parasocial interaction measures was 0.873. The reliability coefficients of all social capital measures were 0.684 for bonding factors and 0.899 for bridging factors. The reliability coefficients of purchase intention of environmentally and socially sustainable product measures were 0.921 and 0.947, respectively. The coefficients indicate acceptable reliability of the measures. This study used partial least squares (PLS) for structural equation modeling, which has good statistical power for samples. Social power, including referent (β = 0.018, p < 0.05) and reward (β = 0.359, p < 0.001), showed statistically positive effects on parasocial interaction. The results partially supported H1. Parasocial interaction showed statistically positive effects on social capital, the bonding factor (β = 0.578, p < 0.001), and the bridging factor (β = 0.651, p < 0.001). Thus, the results supported H2. For parasocial capital, bridging showed statistically positive effects on purchase intention of environmentally (β = 0.233, p < 0.01) and socially (β = 0.284, p < 0.01) sustainable products. Thus, the results partially supported H3 (see Table 1, Figure 1). Discussion This study contributes to clarifying the concept of social capital and determining the relationships between social capital and purchase intention for sustainable fashion products. This study contributes to the theoretical foundation and implications of social capital and sustainability. Specifically, social power, including referent and reward, positively influences parasocial interaction. Parasocial interaction has positive effects on social capital. In turn, social capital positively influences purchase intention for sustainable fashion products. This is the first study on the effects of social capital on purchase intention for sustainable fashion products in the fashion YouTube context. This study suggests that social capital is a strong influential variable for purchase intention regarding sustainable fashion products. Thus, fashion marketers should consider social capital management in the fashion YouTube context while tailoring their brand communications to enhance their sustainable marketing and management.