Setting: Rural Rwandan hospitals, where thermoregulation is critical yet a challenge for pre-term, low-birth-weight (LBW) or sick newborns. Objective: To assess the safety, effectiveness, and feasibility of an inexpensive, reusable, non-electric warmer to complement kangaroo mother care (KMC). Methods: Prospective single-arm, non-randomized intervention study. Enrolled infants were hypothermic or at risk of hypothermia due to prematurity/LBW. Infants used the warmer in conjunction with KMC or as the sole source of external heat. Temperatures of the infant, warmer and air were measured for up to 6 h. Results: Overall, 33 patients used the warmer for 102 encounters: 43 hypothermic and 59 at risk of hypothermia. In 7/102 encounters (7%), the infant developed a temperature of >37.5°C (37.6°-38.2°C). For 43 hypothermic encounters and 59 at-risk encounters, hypothermia was corrected/prevented in respectively 41 (95%) and 59 (100%) instances. The warmer maintained goal temperature for the study duration in ⩾85% of uses. Two/12 warmers broke down after <10 uses. In no instances was the warmer used incorrectly. Conclusion: Our results are promising for this prototype design, and warrant testing on a wider scale.