The transition towards sustainable agriculture necessitates the adoption of eco-friendly alternatives to chemical fertilisers and pesticides. Bio inoculants, comprising beneficial microorganisms, offer significant potential to enhance soil health and crop productivity. However, despite their proven benefits, their adoption among farmers in India remains limited. Existing studies have largely focused on awareness and general adoption patterns, with limited empirical evidence on the combined influence of innovation attributes using advanced modelling approaches such as partial least squares structural equation modelling (PLS-SEM), particularly across diverse agro-climatic regions. This study addresses this gap by examining the determinants of bio-inoculant adoption behaviour among farmers in Tamil Nadu using PLS-SEM based on the innovation diffusion theory (IDT). A total of 240 farmers were selected from eight agro-climatic zones through purposive and snowball sampling techniques. The results indicate that 42.91 % of farmers have adopted bio-inoculants. Relative advantage, compatibility, complexity (low complexity) and observability significantly influence adoption behaviour, whereas trialability has no significant effect. Among these, complexity emerged as the most influential factor, highlighting the importance of ease of use in technology adoption. Despite these positive determinants, key constraints such as lack of awareness, inconsistent product quality and limited availability continue to hinder widespread adoption. The study recommends strengthening extension services, improving quality assurance mechanisms and enhancing farmer awareness through targeted interventions. These findings provide valuable insights for policymakers and extension agencies to promote sustainable agricultural practices through increased adoption of bio-inoculants.