Tobacco smoke has remained a major public health challenge despite numerous strategies devised by international communities to control it. Studies have shown that maternal passive and active smoking is associated with an increased risk of obstetric complications and adverse perinatal outcomes. Parents who choose to smoke are possibly not aware of or deny the negative effects of such action on their offspring. Based on this observation, this study investigated awareness of adverse effects of tobacco smoke on under-5 children amongst nursing mothers attending clinics at Ikotun PHC Lagos. The study employed a descriptive survey design, convenience sampling, a self-designed questionnaire, and both face and content validity to assess the awareness of nursing mothers attending Ikotun Primary Health Centre, Lagos, regarding the adverse effects of tobacco smoke on their under-five children. The collected data were analyzed using SPSS version 23 and presented descriptively in frequency tables to provide insights into the research objectives. Study revealed that the overall awareness and knowledge level of tobacco smoke was good as majority (74.2%) and (52.5%) of the study population were aware and knowledgeable about the harmful effects of tobacco smoke to young children. Also, the nursing mothers exhibited poor perception towards tobacco smoke with a mean score of 1.4 and 37.5% of them that strongly agreed that tobacco smoking is not harmful even during pregnancy. majority (83.3%) and (76.7%) of the study population believed that being a smoker and age respectively affects knowledge and how the pregnant women perceive the adverse effects of tobacco smoking particularly on the unborn. Most (65.8%) affirmed that having a smoker in the family affects the perception of the adverse effects of tobacco smoking. Based on these findings, there is need for public health education programme targeting this population should enhance their self-awareness and consequently increase their knowledge to the complications, discomforts, infertility and adverse perinatal outcomes related to STS exposure and prompt them to adopt prevention strategies.