Abstract Background The emergent COVID-19 has impacted unprecedentedly to all classes of people. Slum-dwellers’ knowledge, attitudes and practices (KAP) toward COVID-19 are currently poorly understood. The present study aimed to investigate the KAP toward COVID-19 among slum dwellers resided in Dhaka City, Bangladesh. Methods A cross-sectional offline survey was carried out enrolling 406 slum dwellers (53.2% male; mean age = 44.9 years [SD = 12.1]; age range = 18–85 years) between August and September, 2020. The face to face interview was conducted to collect data from six selected slum areas in Dhaka City using convenience sampling. The questionnaire consisted of informed consent along with questions concerning observational checklists, socio-demographics and KAP. Results A sizeable minority were observed without wearing face masks during the survey periods (18.2%) and a vast portion (97.5%) without any hand protection. The mean scores of KAP were 6.1 ± 2.6 (out of 17), 12.3 ± 1.7 (out of 14) and 9.8 ± 1.6 (out of 12), respectively. Moreover, the KAP were strongly and positively correlated with each other. Conclusions The findings revealed that the majority of slum dwellers in Bangladesh have limited knowledge of COVID-19. Poor practices (i.e. face mask and hand protection) were directly observed during the survey. The findings suggest the immediate implementation of health education programs and adequate interventions.