Suboptimal breastfeeding is common in Indonesia, with only half of infants 0-5 months of age exclusively breastfed and feeding of breastmilk substitutes (BMS) highly prevalent among infants and toddlers. Various factors influence these feeding practices, including social norms, limited health system support and BMS manufacturer marketing practices. This cross-sectional survey aimed to identify the prevalence of breastfeeding and BMS feeding among children aged 0-35 months, explore socio-demographic characteristics and motivating factors associated with these feeding behaviours and identify the prevalence of mothers' exposure to BMS promotions. Indonesian mothers of children <3 years of age (n = 595) were interviewed in Bandung City health facilities using structured questionnaires. Although all children were ever breastfed, half of children across all age groups received BMS in the previous day. Maternal employment outside the home and insufficient breastmilk production were associated with BMS use. The most important motivational factors for feeding BMS were perceived benefits for growth, intelligence and immunity. Despite Indonesian legislation restricting some BMS marketing, 93% of mothers reported observing a BMS promotion outside the health system, with television, social media and newspapers as the most common sources. Half of mothers (43%) reported observing a BMS promotion within the health system, and half (46%) reported receiving recommendations from health workers to use BMS. Such high prevalence of BMS marketing may be influencing caregivers' feeding choices; stronger national legislation and implementation of laws are needed to ensure mothers' ability to make feeding choices free from manufacturer influence.