Abstract Background Although breastfeeding is almost universally accepted in the Democratic Republic (DR) of Congo, by the age of 2 to 3 months 65% of children are receiving something other than human milk. We sought to describe the infant feeding practices and determinants of suboptimal breastfeeding behaviors in DR Congo. Methods Survey questionnaire administered to mothers of infants aged ≤ 6 months and healthcare providers who were recruited consecutively at six selected primary health care facilities in Kinshasa, the capital. Results All 66 mothers interviewed were breastfeeding. Before initiating breastfeeding, 23 gave their infants something other than their milk, including: sugar water (16) or water (2). During the twenty-four hours prior to interview, 26 (39%) infants were exclusively breastfed (EBF), whereas 18 (27%), 12 (18%), and 10 (15%) received water, tea, formula, or porridge, respectively, in addition to human milk. The main reasons for water supplementation included “heat” and cultural beliefs that water is needed for proper digestion of human milk. The main reason for formula supplementation was the impression that the baby was not getting enough milk; and for porridge supplementation, the belief that the child was old enough to start complementary food. Virtually all mothers reported that breastfeeding was discussed during antenatal clinic visit and half reported receiving help regarding breastfeeding from a health provider either after birth or during well-child clinic visit. Despite a median of at least 14 years of experience in these facilities, healthcare workers surveyed had little to no formal training on how to support breastfeeding and inadequate breastfeeding-related knowledge and skills. The facilities lacked any written policy about breastfeeding. Conclusion Addressing cultural beliefs, training healthcare providers adequately on breastfeeding support skills, and providing structured breastfeeding support after maternity discharge is needed to promote EBF in the DR Congo.