AbstractRelapsed/refractory (r/r) acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) outcomes remain poor. A targeted cluster of differentiation (CD)33 × CD3 bispecific antibody, JNJ‐67571244, was assessed to identify the maximum tolerated dose (MTD), recommended phase II dose (RP2D), safety and tolerability, and preliminary clinical activity in patients with r/rAML or r/rMDS. This first‐in‐human, open‐label, phase I, dose‐escalation/dose‐expansion study included patients with r/rAML or r/rMDS who were ineligible for or had exhausted standard therapeutic options. JNJ‐67571244 was administered intravenously or subcutaneously using step‐up dosing until ≥1 discontinuation condition was met. Outcomes included safety/tolerability, preliminary clinical activity, and systemic pharmacokinetics and pharmacodynamics. The study was terminated after evaluating 10 dose‐escalation cohorts (n = 68) and before starting dose‐expansion. Overall, 11 (16.2%) patients experienced ≥1 dose‐limiting toxicity; all experienced ≥1 treatment‐emergent adverse event (TEAE; treatment related: 60 [88.2%]); and 64 (94.1%) experienced ≥1 TEAE of Grade ≥3 toxicity (treatment related: 28 [41.2%]). Although some patients had temporary disease burden reductions, no responses were seen. JNJ‐67571244 administration increased multiple cytokines, which coincided with incidence of cytokine release syndrome, infusion‐related reactions, and elevated liver function tests. A prolonged step‐up strategy was tested to improve tolerability, though this approach did not prevent hepatotoxicity. T‐cell activation following treatment suggested target engagement but did not correlate with clinical activity. Safely reaching the projected exposure level for JNJ‐67571244 efficacy was not achieved, thus MTD and RP2D were not determined.