Background: The presence of diabetes in patients with heart failure (HF) is associated with a worse prognosis. It is unclear if hemodynamics in HF patients with DM differ from those of non-diabetic patients and how this might influence outcome. This study aims to discover the impact of DM on hemodynamics in HF patients. Methods: Consecutive patients (n = 598) with HF and reduced ejection fraction (LVEF ≤40%) undergoing invasive hemodynamic evaluation were included (non-DM: n = 473, DM: n = 125). Hemodynamic parameters included pulmonary capillary wedge pressure (PCWP), central venous pressure (CVP), cardiac index (CI) and mean arterial pressure (MAP). Mean follow-up was 9.5 ± 5.1 years. Results: Patients with DM (82.7% male, mean age 57.1 ± 10.1 years, mean HbA1c 60 ± 21 mmol/mol) had higher PCWP, mPAP, CVP and higher MAP. Adjusted analysis demonstrated that DM patients had higher PCWP and CVP. Increasing HbA1c-values were correlated with higher PCWP (p = 0.017) and CVP (p = 0.043). Conclusion: Patients with DM, especially those with poor glycemic control, have higher filling pressures. This may be a feature of diabetic cardiomyopathy, however, other unknown mechanisms beyond hemodynamic factors are likely to explain the increased mortality associated with diabetes in HF.