This is a 2-part, open-label, multicenter, Phase 2/3 study conducted in participants with AML who are suitable for experimental therapy (Part A) and in participants with untreated AML who are not eligible for intense induction chemotherapy or hematopoeitic stem cell transplantation (HSCT) (Part B). In Study Part A, the safety, pharmacokinetic (PK) and pharmacodynamic (PD) profile will be assessed to confirm the RP2D of 9 milligram per kilogram (mg/kg) talacotuzumab. In Study Part B, participants will be randomized in a 1:1 ratio into either decitabine + talacotuzumab (arm 1) or decitabine alone (arm 2). Blood and bone marrow sampling will be done in Part A and B for disease assessment, PK, PD, and biomarkers will be collected in all participants. Safety will be monitored throughout the study.