This study aimed to identify the knowledge, attitudes, and practices related to the prevention of early childhood caries (ECC) amongst medical personnel in well-baby clinics (WBC). Participants were 152 health care professionals, including public health officers, nurses, and physicians, working in WBC in Chonburi Province, Thailand. Participants completed a 6-part online questionnaire asking about demographic data, knowledge and attitudes related to ECC, ECC prevention practices, collaboration, referral systems, and suggestions for improving collaboration between dental and medical personnel in WBC. Whilst most participants had high levels of knowledge about oral care instructions, tooth brushing frequency, and cariogenic foods and beverages, they knew less about recommendations for timing of the first dental visit, fluoride concentration in toothpaste, and the time for exfoliation of primary teeth. There were significant differences in the proportions of correct answers to questions related to the first dental visit and bottle feeding and in total knowledge scores between physicians and other health care professionals. Most participants demonstrated a positive attitude towards ECC prevention. More than 70% of participants always advise parents on oral hygiene and nutrition related to ECC prevention, but only 29% always refer patients to see dental personnel. Attitudes, collaboration, and referral systems were significantly associated with ECC prevention practices, but not knowledge. This study demonstrates that attitudes, collaboration, and referral systems are key contributors to ECC prevention amongst Thai medical personnel. In addition to training programmes, consistent interprofessional policy and referral systems should be formulated, especially in primary care settings.