The goal of this cross-sectional survey was to assess the level of knowledge on harmful effects of environmental tobacco smoke (ETS) exposure and active smoking among socially-disadvantaged people in Poland. The study was conducted among 1817 respondents aged 18–59 years, who used aid services from local social care institutions in Piotrkowski district. Majority of the participants were aware of the fact that smoking may cause serious diseases and lung cancer (92%). However, those percentages were lower for awareness of ETS and health risk (69.4%) and for awareness of smoking/ETS-associated risk of stroke and heart attack (57%, 68%). The male respondents and smokers had much higher odds of lacking knowledge that smoking causes serious diseases and lung cancer compared to the females (OR = 1.47 and OR = 1.86; p < 0.05) and non-smokers (OR = 2.35 and OR = 2.31; p < 0.001). In addition, those with temporary jobs and the unemployed had a higher risk of lack of knowledge on smoking and lung cancer risk (OR = 2.14 and OR = 1.66; p < 0.05) as well as ETS and the risk of stroke (OR = 1.52 and OR = 1.51; p < 0.05) as compared to those with permanent jobs. The smokers who were aware of four health consequences of smoking indicated an intention to quit smoking within the next month more frequently when compared to those who did not have the knowledge on all of the analyzed harmful effects of tobacco use (19.7% vs. 13.1%; p < 0.05). There is a need to improve knowledge on the dangers associated with active and passive smoking among socially disadvantaged populations.