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Vitamin D emerged as an important prognostic biomarker in heart failure (HF), with currently highly debated therapeutic implications. Several trials on vitamin D supplementation in HF showed improvements in left ventricular (LV) remodeling and function and health-related quality of life (HRQoL), which did not translate into mid- to long-term beneficial effects regarding physical performance and mortality. We addressed total 25-hydroxyvitamin D (25(OH)D), serum albumin, and uric acid (UA) levels, focusing mainly on vitamin D deficiency, as potential markers of LV systolic dysfunction in HF with reduced and mildly reduced ejection fraction (HFrEF, HFmrEF). Seventy patients with LVEF < 50% were comprehensively evaluated using ECG, echocardiography, lung ultrasound (LUS), blood sampling, and the six-minute walk test (6MWT). HRQoL was also assessed using the Minnesota Living with Heart Failure Questionnaire (MLHFQ). Statistically significant positive correlations were found between LVEF, 25(OH)D, serum UA, and albumin, respectively (p = 0.008, p = 0.009, and p = 0.001). Serum UA (7.4 ± 2.4 vs. 5.7 ± 2.1, p = 0.005), NT-proBNP levels (1090.4 (675.2–2664.9) vs. 759.0 (260.3–1474.8), p = 0.034), and MLHFQ scores (21.0 (14.0–47.0) vs. 14.5 (4.5–25.5), p = 0.012) were significantly higher, whereas 25(OH)D concentrations (17.6 (15.1–28.2) vs. 22.7 (19.5–33.8), p = 0.010) were lower in subjects with severely reduced LVEF. Also, 25(OH)D was independently associated with LVEF in univariate and multiple regression analysis, maintaining its significance even after adjusting for confounders such as age, NT-proBNP, the presence of chronic coronary syndrome, hypertension, and anemia. According to our current findings, 25(OH)D is closely associated with LVEF, further supporting the need to establish correct vitamin D supplementation schemes and dietary interventions in HF. The changes in LVEF, 25(OH)D, serum UA, and albumin levels in HFrEF and HFmrEF indicate a similar pathophysiological background.