User-generated content (UGC) has restructured how destination brands are formed, but the tourism-marketing literature has largely theorised this shift in the context of mainstream, high-volume destinations, leaving the branding dynamics of responsible tourism (RT) destinations under-conceptualised. This paper addresses that omission by developing an integrative conceptual framework explaining how UGC contributes to destination branding in Kerala's RT destinations. Rather than treating relevant theories in isolation, the framework fuses Source Credibility Theory, electronic word-of-mouth (eWOM) theory, destination branding theory, and brand co-creation theory into a single explanatory chain. It positions UGC credibility and UGC authenticity as the qualities that make peer-generated eWOM persuasive; it treats responsible tourism attributes — community participation, cultural authenticity, environmental sustainability, local products, and ethical tourism practices — as the contextual brand meanings that credible, authentic content renders believable in a domain otherwise vulnerable to greenwashing scepticism; and it models destination brand image as the mediating asset that converts co-created content into travel intention. Six propositions specify the relationships among these constructs and are justified through the integrated theoretical logic. The paper contributes a theory-grounded account of how tourist-authored content co-creates a responsible destination brand, extends credibility and co-creation reasoning into the sustainability domain, and offers destination management organisations guidance on curating authentic advocacy rather than manufacturing it.