In today’s digital era, where celebrities hold immense influence over consumer behavior, understanding the factors that shape trust and online purchase toward endorsed brands is crucial. Especially, in the landscape of celebrity endorsement, the effectiveness of celebrity authenticity, online celebrity fandom, and their interaction has been fruitful yet underexplored research direction. Drawing upon signaling theory and trust transfer mechanism, we conducted a 2×2 between-subjects factorial design experiment to examine how signals from celebrity authenticity and sense of online fandom affect trust development and subsequent online purchase among consumers. Data from 200 participants were analyzed using MANCOVA and PLS-SEM. Our findings indicate that high celebrity authenticity and a sense of healthy fandom lead to greater wishful identification and celebrity trust. Additionally, the effects of perceived celebrity authenticity on wishful identification and celebrity trust are stronger in the presence of a sense of healthy fandom compared to toxic fandom. Furthermore, wishful identification directly influences consumers’ online purchase intentions, while brand trust fully mediates the effect of celebrity trust on purchase intention. This original study pioneers experimental research on the joint effects of celebrity authenticity and online fandom, advancing signaling theory and trust transfer literature within the e-commerce and celebrity endorsement context.