Bánh mì has become an internationally recognized symbol of Vietnamese culinary identity, yet its authenticity is increasingly constructed and negotiated through digital media. This study examines how TikTok User-Generated Content (UGC) constructs bánh mì’s authenticity and contributes to Vietnam’s digital gastrodiplomacy. Drawing on cultural hybridity, cultural identity, and encoding/decoding perspectives, the study combines multimodal analysis of 204 TikTok videos with a survey of 312 respondents. Findings show that authenticity is constructed through material, cultural, experiential, and performative dimensions, reflecting Vietnamese adaptation and the negotiation of local and global meanings. TikTok UGC also functions as grassroots gastrodiplomacy, enhancing bánh mì’s visibility and association with Vietnamese culinary identity. However, limited historical and cultural narratives may reduce authenticity to sensory and visual markers. The study argues that sustaining bánh mì’s authenticity requires balancing global adaptation and innovation with the preservation of its distinctive Vietnamese cultural meanings.