Objective: This work proposes a model of formation of brand extension loyalty of a heritage brand parent based on the attitude towards the heritage brand parent, taking into account the mediating effect of the brand parent image and the moderating effects of the fit between the heritage parent brand and the brand extension as well as the involvement towards the product category. Methodology: An experimental study has been carried out with 328 tourists visiting the heritage destination and manipulating two levels of adjustment. A moderate mediation regression model was established using the PROCESS 3.4 software. Conclusions: The effect of the attitude towards the heritage brand on the loyalty towards the extension occurs through the image of the heritage brand. This effect is enhanced in low setting and high involvement conditions. The involvement moderates the direct effect regardless of the degree of brand extension fit. Implications: Implications are provided for the management of companies that use a heritage brand to compete in the market as well as for managers of cultural heritage. Originality and value: This work contributes to the existing knowledge about the process to the formation of brand-extension loyalty of brand of cultural heritage site, taking into account the perceptions about the parent brand in terms of attitude and image. Also, this study examines the moderating role of brand extension fit and the level of involvement with the category.