Aim: This study aims to examine the influence of formula milk promotion via the media from Thailand to Lao People's Democratic Republic (PDR), where a cultural and linguistic proximity are shared. Methods: A cross-sectional study was conducted through a structured questionnaire survey and focus group discussion (FGD) with mothers who had children under 2 years of age and lived in Vientiane Capital, Lao PDR. Multivariate logistic regression models were constructed for quantitative data analysis. Content analysis was used for qualitative data analysis. Results: Among infants aged 6-23 months, exclusive breastfeeding (EBF) rate for 6 months was 16.1% (n = 106/658). Among infants aged 0-5 months, 17.6% (n = 61/346) was exclusively breastfed at the time of survey (24 h recall). Of 1022 mothers, 89.9% reported frequent exposure to the Thai media's promotion of formula milk through TV commercials and 79.1% identified TV commercial as influential for them to develop a positive attitude towards the use of formula milk. In multivariate logistic regression analyses, mothers who reported a positive attitude towards Thai TV commercial on the formula use (n = 449) were approximately 75% less likely to practice EBF for 6 months than those who reported a negative attitude (n = 64). FGD further revealed that the participants tend to believe in the information in TV commercial for formula milk. Conclusion: The promotion of formula milk via media from Thailand negatively affects breastfeeding mothers in Lao PDR. Cross-border impacts of promoting formula milk should be addressed globally.