Background and aim: Heart failure (HF) and diabetes mellitus (DM) are burdensome chronic diseases with high lifetime risks and numerous studies indicate associations between HF and DM. The objective of this study was to investigate the direct and indirect costs of HF patients with and without DM. Methods and results: Patients with a first-time diagnosis of HF from 1998 to 2016 were identified through nationwide Danish registries and stratified according to DM status into HF with or without DM. The economic healthcare cost analysis was based on both direct costs, including hospitalization, procedures, medication and indirect costs including social welfare and lost productivity. The economic burden was investigated prior to, at, and following diagnosis of HF. Patients with concomitant HF and DM were younger (median age 74 vs. 77), had more comorbidities and fewer were female as compared to patients with HF but without DM. The socioeconomic burden of concomitant HF and DM compared to HF alone was substantially higher; 45% in direct costs (€16,237 vs. €11,184), 35% in home care costs (€3123 vs. €2320), 8% in social transfer income (€17,257 vs. €15,994) and they had 27% lower income (€10,136 vs. €13,845). The economic burden peaked at year of diagnosis, but the difference became increasingly pronounced in the years following the HF diagnosis. Conclusion: Patients with concomitant HF and DM had a significantly higher economic burden compared to patients with HF but without DM.