The World Health Organization defines low birth weight (LBW) as a new born having a weight of less than 2,500 g at birth. Low birth weight is one of the major determinants of perinatal survival, infant morbidity and mortality as well as the risk of developmental disabilities and illnesses in future lives. WHO estimates that about 30 million low birth weight babies are born annually (23.4% of all births) and they often face short and long term health consequences. Whereas the global prevalence of LBW has slightly declined, the rate in many developing countries is still quite high. In Uganda, low birth weight among teenage mothers is a problem. Our study aimed to estimate the prevalence of and identify the factors associated with low birth weight among teenage mothers in New Mulago hospital. We conducted an analytical cross sectional study among teenage mothers who delivered from new Mulago Hospital Complex labour suite from August 2013 to August 2014. Trained interviewers, administered pre-tested questionnaires to consecutive mothers to obtain information on their socio-demographic characteristics, obstetric history and child factors. Odds ratios and P-values were calculated to determine the relationship between independent and dependent variables. We also used descriptive statistics for the quantitative data. A total of 357 teenage mothers were enrolled on the study. Their mean age was 18 years (Range 13–19), majority, 98.4% aged 15–19 years. The prevalence of LBW was 25.5%. Pre-term delivery (OR = 3.3032 P = 0.0001) and multiple pregnancies (OR = 0.165 P = 0.039) were associated with LBW. Malaria, young maternal age and ANC attendance were not associated with LBW. Child factors such as birth order, congenital anomalies and sex of the baby were also not associated with LBW. The prevalence of LBW is high among teenage mothers, pre-term delivery and multiple pregnancies were associated factors with LBW. Health professional’s need to address teenage maternal health. Health workers should encourage teenage mothers to attend focused antenatal care as recommended by the Uganda ministry of Health. A specialized maternal facility centre that is friendly for adolescent/teenage mothers is advisable so as to improve on completion rates and capture high risk teenage mothers early.