Over the past decade, reducing 30-day readmission rates has been emphasized in the United States (including via the implementation of the Hospital Readmissions Reduction Program) but not Canada. To examine changes that occurred from April 1, 2005, to December 31, 2015, in the United States and Canada for hospitalization length of stay and 30-day readmission rates of patients with heart failure. This cohort study included patients admitted with a primary diagnosis of heart failure to Canadian and US hospitals between April 1, 2005, and December 31, 2015, using International Classification of Diseases, Ninth Revision code 428.xx and Tenth Revision code I50. The study examined secular trends in length of stay and readmissions in both countries and tested for changes after implementation of the Hospital Readmissions Reduction Program using segmented regression models and the association between length of stay and readmissions using patient-level and hospital-level multivariable logistic regression models. Data analysis was completed from February 2018 to August 2018. Thirty-day readmissions. Between 2005 and 2015, mean length of stay declined marginally in Canadian hospitals (from a mean [SD] of 7.5 [5.7] to 7.3 [5.6] days; P < .001) but remained stable in US hospitals (mean [SD], 4.9 [3.7] days to 4.9 [3.5] days). Thirty-day readmission rates declined similarly in Canada (from 4088 of 20 758 patients [19.7%] to 3823 of 21 733 patients [17.6%] for all-cause readmissions; P < .001; and from 1743 of 20 758 patients [8.4%] to 1490 of 21 733 patients [6.9%] for heart failure-specific readmissions; P < .001) and the United States (from 21.2% to 18.5% for all-cause readmissions; from 7.6% to 5.7% for heart failure-specific readmissions; both P < .001). There were small but statistically significant positive correlations between length of stay and 30-day readmissions in both Canada (odds ratio, 1.01 [95% CI, 1.01-1.01]) and the United States (odds ratio, 1.01 [95% CI, 1.01-1.01]). Interrupted time-series analysis comparing readmission rates before and after the Hospital Readmissions Reduction Program implementation revealed no significant difference in either country for all-cause readmission rates before and after October 2012. There was also no change in the slope of the temporal trends; in Canada, all-cause readmissions were decreasing 1.1% per year before implementation and 1.3% after implementation (P = .84 for slope change) compared with 1.6% per year in the United States before implementation and 1.8% per year after October 2012 (P = .60 for slope change). Both Canada and the United States exhibited similar temporal declines in 30-day all-cause readmissions over the past decade. These findings suggest that the Hospital Readmissions Reduction Program did not appear to be associated with this secular trend or length of stay for heart failure in the United States.