In our retrospective study, we aimed to investigate the relationship between urinary chloride (uCl⁻) and selected clinical and laboratory biomarkers, renal function, and patient outcomes in the acute heart failure (AHF) population. We divided 248 adult patients (≥ 18 years) with AHF into two groups: low uCl⁻ (< 115 mmol/L) and high uCl⁻. The mean age of the patient group was 70.2 ± 12.6, and 182 patients were male (73.4%). Clinical endpoints included in-hospital mortality, one-year mortality, and a composite endpoint of one-year mortality and rehospitalization for heart failure. Patients were followed up for at least one year. Relevant clinical and baseline biomarker data were collected, including markers concerning inflammation, liver and kidney function, perfusion and congestion, iron status, cardiac remodeling, gasometry, renin and aldosterone. Low uCl⁻ was associated with worse in-hospital outcomes, including higher in-hospital mortality (7.7% vs. 1.4%, p = 0.014), the need for inotropic support (20.19% vs. 2.08%, p ≤ 0.001), worsening of HF during therapy (17.31% vs. 4.86%, p ≤ 0.001), and the need for treatment in an intensive cardiac care unit (33.65% vs. 15.28%, p ≤ 0.001). Low uCl⁻ was a significant predictor of one-year mortality (40.4% vs. 16.7%, p < 0.05) and the composite outcome (HR 2.42, 95% CI 1.43–4.08, p < 0.001). In the multivariable analysis, uCl⁻ was independently associated with the risk of one-year mortality (HR 0.92, 95% CI 0.87–0.98, p < 0.05) and the composite outcome (HR 0.95, 95% CI 0.92–0.99, p < 0.05). Our findings suggest that low uCl⁻ is a marker of more advanced heart failure, activation of the renin–angiotensin–aldosterone system and is related to worse one-year outcomes.