Objective To investigate the relationship between ambient temperature and atmospheric pressure (AP) and the severity of heart failure (HF) decompensations. Methods We analysed patients coming from the Epidemioloy Acute Heart Failure Emergency (EAHFE) Registry, a multicentre prospective cohort study enrolling patients diagnosed with decompensated HF in 26 emergency departments (EDs) of 16 Spanish cities. We recorded patient and demographic data and maximum temperature ( T max ) and AP (AP max ) the day before ED consultation. Associations between temperature and AP and severity endpoints were explored by logistic regression. We used restricted cubic splines to model continuous non-linear associations of temperature and AP with each endpoint. Results We analysed 16,545 patients. Daily T max and AP max (anomaly) of the day before patient ED arrival ranged from 0.8 to 41.6° and from − 61.7 to 69.9 hPa, respectively. A total of 12,352 patients (75.2%) were hospitalised, with in-hospital mortality in 1171 (7.1%). The probability of hospitalisation by HF decompensation showed a U-shaped curve versus T max and an increasing trend versus AP max . Regarding temperature, hospitalisation significantly increased from 20 °C (reference) upwards (25 °C: OR = 1.12, 95% CI = 1.04–1.21; 40 °C: 1.65, 1.13–2.40) and below 5.4 °C (5 °C: 1.21, 1.01–1.46). Concerning the mean AP of the city (anomaly = 0 hPa), hospitalisation increased when AP max (anomaly) was above + 7.0 hPa (atmospheric anticyclone; + 10 hPa: 1.14, 1.05–1.24; + 30 hPa: 2.02. 1.35–3.03). The lowest probability of mortality also corresponded to cold-mild temperatures and low AP, with a significant increased risk only found for T max above 24.3 °C (25 °C: 1.13, 1.01–1.27; 40 °C: 2.05, 1.15–3.64) and AP max (anomaly) above + 3.4 hPa (+ 10 hPa: 1.21, 1.07–1.36; + 30 hPa: 1.73, 1.06–2.81). Sensitivity analysis confirmed the main analysis results. Conclusion Temperature and AP are independently associated with the severity of HF decompensations, with possible different effects on the need for hospitalisation and in-hospital mortality.