Objective Rectal cancer is common in developed countries, though incidence varies globally. We assessed time trends in incidence, relative survival and mortality in Denmark. Methods Rectal cancer cases (N = 50 461) diagnosed in 1978-2018 were identified in the Danish Cancer Registry. We calculated age-standardized incidence rates, overall and according to sex and age. Average annual percentage changes (AAPC) were estimated using Poisson regression. We estimated 5-year relative survival and evaluated the effect of age, calendar year of diagnosis, sex and stage of disease on mortality using the Cox proportional hazards model. Results The incidence of rectal cancer tended to decrease in all age groups and both sexes during 1978-1997, but increased since 1998, more in men (AAPC = 2.05%; 95% CI,1.80; 2.31) than in women (AAPC = 0.99%; 95% CI,0.68; 1.30). It increased in men until 79 years and in women up to 59 years. Mortality decreased over time when adjusting for age, stage and sex. Overall, men had the highest 5-year mortality after adjusting for age, calendar period and stage. Five-year relative survival improved (1978-2018) for all stages. Initially, the overall 5-year relative survival tended to be better for women, but in recent years, it has been similar in both sexes. Conclusion Incidence of rectal cancer increased in the last two decades, most markedly in women 59 years and younger. Mortality decreased when adjusting for age and stage. Relative survival improved over time more for men than for women, so in recent years, it has been virtually identical in men and women.