Purpose: To examine self-reported (30-day) sleep versus nightly actigraphy-assessed sleep concordance in long-term survivors of childhood cancer. Methods: Four hundred seventy-seven participants enrolled in the St. Jude Lifetime Cohort (53.5% female, median (range) age 34.3 (19.3–61.6) years, 25.4 (10.9–49.3) years from diagnosis) completed the Pittsburgh Sleep Quality Index and ≥ 3 nights of actigraphy. Participants had neurocognitive impairment and/or a self-reported prolonged sleep onset latency (SOL). Self-reported 30-day sleep and nightly actigraphic sleep measures for sleep duration, SOL, and sleep efficiency (SE) were converted into ordinal categories for calculation of weighted kappa coefficients. General linear models estimated associations between measurement concordance and late effects. Results: Agreements between self-reported and actigraphic measures were slight to fair for sleep duration and SOL measures (kw = 0.20 and kw = 0.22, respectively; p < 0.0001) and poor for SE measures (kw = 0.00, p = 0.79). In multivariable models, severe fatigue and poor sleep quality were significantly associated with greater absolute differences between self-reported and actigraphy-assessed sleep durations (B = 26.6 [p < 0.001] and B = 26.8 [p = 0.01], respectively). Survivors with (versus without) memory impairment had a 44-min higher absolute difference in sleep duration (B = 44.4, p < 0.001). Survivors with, versus without, depression and poor sleep quality had higher absolute discrepancies of SOL (B = 24.5 [p = 0.01] and B = 16.4 [p < 0.0001], respectively). Poor sleep quality was associated with a 12% higher absolute difference in SE (B = 12.32, p < 0.0001). Conclusions: Self-reported sleep and actigraphic sleep demonstrated discordance in our sample. Several prevalent late effects were statistically significantly associated with increased measurement discrepancy. Future studies should consider the impacts of late effects on sleep assessment in adult survivors of childhood cancer.