Objective To investigate the association of corrected QT (QTc) interval duration and short-term outcomes in patients with acute heart failure (AHF). Methods We analyzed AHF patients enrolled in 11 Spanish emergency departments (ED) for whom an ECG with QTc measurement was available. Patients with pace-maker rhythm were excluded. Primary outcome was 30-day all-cause mortality and secondary outcomes were need of hospitalization, in-hospital mortality and prolonged hospitalization (> 7 days). Association between QTc and outcomes was explored by restricted cubic spline (RCS) curves. Results were expressed as odds ratios (OR) and 95%CI adjusted by patients baseline and decompensation characteristics, using a QTc = 450 ms as reference. Results Of 1800 patients meeting entry criteria (median age 84 years (IQR = 77–89), 56% female), their median QTc was 453 ms (IQR = 422–483). The 30-day mortality was 9.7%, while need of hospitalization, in-hospital mortality and prolonged hospitalization were 77.8%, 9.0% and 50.0%, respectively. RCS curves found longer QTc was associated with 30-day mortality if > 561 ms, OR = 1.86 (1.00–3.45), and increased up to OR = 10.5 (2.25–49.1), for QTc = 674 ms. A similar pattern was observed for in-hospital mortality; OR = 2.64 (1.04–6.69), for QTc = 588 ms, and increasing up to OR = 8.02 (1.30–49.3), for QTc = 674 ms. Conversely, the need of hospitalization had a U-shaped relationship: being increased in patients with shorter QTc OR = 1.45 (1.00–2.09) for QTc = 381 ms, OR = 5.88 (1.25–27.6) for the shortest QTc of 200 ms, and also increasing for prolonged QTc OR = 1.06 (1.00–1.13), for QTc = 459 ms, and reaching OR = 2.15 (1.00–4.62) for QTc = 588 ms. QTc was not associated with prolonged hospitalization. Conclusion In ED AHF patients, initial QTc provides independent short-term prognostic information, with increasing QTc associated with increasing mortality, while both, shortened and prolonged QTc are associated with need of hospitalization. Graphical abstract