Electronic cigarettes (e-cigarettes) have gained popularity as a less harmful alternative to conventional cigarettes, yet their associations with lung cancer risk after smoking cessation remain uncertain. Here we evaluated 4,524,895 adults with a conventional smoking history who participated in the Korean National Health Screening Program in 2018 (baseline), with prior records from 2012-2014. Participants were classified as current smokers, short-term quitters or long-term quitters, and followed up to December 2023. Daily e-cigarette use at baseline was used to define post-cessation e-cigarette use. Lung cancer incidence and lung cancer-specific death (LCSD) were assessed using multivariable Cox models. Over 24,182,543 person-years, 35,887 lung cancers and 12,807 LCSD events occurred. Compared with complete quitters, e-cigarette use after smoking cessation was associated with higher risks of lung cancer incidence (adjusted hazard ratio (aHR) 1.56, 95% confidence interval (CI) 1.24-1.97) and LCSD (aHR 2.00, 95% CI 1.28-3.15). Associations were directionally consistent in short-term and long-term quitters and were prominent in the high-risk subgroup (incidence: aHR 1.91, 95% CI 1.44-2.53; LCSD: aHR 1.92, 95% CI 1.13-3.24). Although causality cannot be established, these findings suggest that e-cigarette use after smoking cessation may attenuate the benefits of complete cessation for lung cancer prevention.