In 2019, 14.0% of infants in Thailand were exclusively breastfed up to 6 months of age. To increase exclusive breastfeeding, an objective assessment measure would be useful to maternity care providers for appraising the problems encountered by new mothers' experiences with breastfeeding. To translate the Bristol Breastfeeding Assessment Tool from English into Thai; to assess the reliability and validity of the Thai Bristol Breastfeeding Assessment Tool; and to explore the relationship of a mother's self-efficacy to successful breastfeeding. Using a methodological design, we purposively sampled 302 new mothers from two tertiary hospitals in Thailand who had given birth to a single baby with an uncomplicated vaginal birth. The Thai Bristol Breastfeeding Assessment Tool's structure was examined using confirmatory factor analysis. Concurrent and convergent validity were assessed using the Thai LATCH instrument and Thai Breastfeeding Self-Efficacy Scale, respectively. The inter-item Thai Bristol Breastfeeding Assessment Tool correlation coefficients were positive and strong. The correlation between scores of the Thai Bristol Breastfeeding Assessment Tool and Thai LATCH instrument were positive and strong and had acceptable concurrent validity. The confirmatory factor analysis model fit the Thai Bristol Breastfeeding Assessment Tool data perfectly and the Cronbach's alpha coefficient had acceptable internal reliability. The Thai Bristol Breastfeeding Assessment Tool is a cross-culturally translated instrument equivalent to its English version and demonstrated acceptable validity and reliability. The instrument provides a mechanism for an objective assessment and monitoring system of optimal breastfeeding practices in Thai mothers with newborns.