Recently, some fashion retailers have initiated sustainable actions in the form of corporate social responsibility (CSR) activities to address consumers’ social concerns. This study intends to combine the concept of CSR motivation attributions with brand extension literature in order to examine how consumers’ trust of a parent brand affects their CSR motivation attributions and ultimately their attitudes towards parent-brand CSR activities and sustainable extension lines. A self-administered online survey was conducted using scenarios describing a fast fashion retailer introducing a sustainable line. Data from 303 female U.S. consumers, aged between 18 and 34 years, were used for the statistical analysis. Data analyses were conducted using SPSS 21.0 for descriptive analysis, exploratory factor analysis, and regression analysis. The results revealed that all hypotheses were supported. The findings show that consumers’ trust of a parent brand has an indirect impact on positive attitudes towards the parent brand’s CSR activities as well as the sustainable extension product line, mediating positive evaluations of the brand’s social CSR motivations. In addition, this study suggests consumers’ prior trust of the parent brand is a variable that affects consumers’ evaluation of CSR motivation sincerity. Marketing suggestions and thoughts based on the findings of this study were provided.