Abstract Secondary acute myeloid leukemia (sAML) comprises a heterogeneous group of patients and is associated with poor overall survival (OS). We analyze the characteristics, treatment patterns, and outcomes of adult patients with sAML in the Programa Español de Tratamientos en Hematología (PETHEMA) registry. Overall, 6211 (72.9%) were de novo and 2310 (27.1%) had sAML, divided into myelodysplastic syndrome AML (MDS-AML, 44%), MDS/myeloproliferative AML (MDS/MPN-AML, 10%), MPN-AML (11%), therapy-related AML (t-AML, 25%), and antecedent neoplasia without prior chemotherapy/radiotherapy (neo-AML, 9%). Compared with de novo, patients with sAML were older (median age, 69 years), had more Eastern Cooperative Oncology Group ≥2 (35%) or high-risk cytogenetics (40%), less FMS-like tyrosine kinase 3 internal tandem duplication (11%), and nucleophosmin 1 (NPM1) mutations (21%) and received less intensive chemotherapy regimens (38%) (all P < .001). Median OS was higher for de novo than sAML (10.9 vs 5.6 months; P < .001) and shorter in sAML after hematologic disorder (MDS, MDS/MPN, or MPN) compared with t-AML and neo-AML (5.3 vs 6.1 vs 5.7 months, respectively; P = .04). After intensive chemotherapy, median OS was better among patients with de novo and neo-AML (17.2 and 14.6 months, respectively). No OS differences were observed after hypomethylating agents according to type of AML. sAML was an independent adverse prognostic factor for OS. We confirmed high prevalence and adverse features of sAML and established its independent adverse prognostic value. This trial was registered at www.clinicaltrials.gov as #NCT02607059.