An extended theory of planned behaviour (TPB) model of cycling tourism was proposed to explore relations among constructs of the model with the addition of the attractiveness of sustainable alternatives and the actual behaviour of cycling tourists in the context of Thai cyclists. Through the analysis of 395 bicycle tourists by structural equation modelling technique, this study extended the TPB model, utilising attitude, subjective norm, and perceived behavioural control towards cycling tourism as predictors and the attractiveness of sustainable alternatives as a mediator. The findings revealed that attitude towards behaviour, subjective norm, perceived behavioural control, behavioural intention, actual behaviour, and attractiveness of sustainable alternatives were significantly supported, and that attractiveness of sustainable alternatives significantly mediated the impact of behavioural intention on the actual behaviour of cycling tourism. Overall, this study helps us understand the role of volitional and non-volitional processes, behavioural intention, actual behaviour, and the attractiveness of sustainable alternatives in explaining the intention formation of cycling tourism in Thailand.