ABSTRACTAficamten is a next‐in‐class, small‐molecule, cardiac myosin inhibitor in development for treating hypertrophic cardiomyopathy (HCM). This 2‐part study evaluated aficamten's impact on QTc interval in healthy participants. Part A (n = 10) was an open‐label study to find the appropriate dose for thorough QT (TQT) evaluation in Part B. Part B (n = 34) was a double‐blind, 3‐way crossover TQT study conducted as per ICH E14 guidance using negative (placebo) and positive (moxifloxacin) controls. A single 50 mg aficamten dose achieved exposures (Cmax range: 124–1660 ng/mL) comparable to the highest clinical dose (20 mg QD) in obstructive HCM patients (NCT05186818; [Cmax range: 131–1230 ng/mL]) and was chosen for TQT evaluation. Using concentration‐QT (C‐QT) modeling, the placebo‐ and baseline‐corrected QT interval using Fridericia's correction (ΔΔQTcF) was −1.82 msec (90% CI: −3.43, −0.214) at peak aficamten concentrations (298.3 ng/mL) following the 50 mg dose. The 90% CI upper bound of ΔΔQTcF for aficamten was < 10 msec at all post‐dose time points. Assay sensitivity was established by the 90% CI lower bound for moxifloxacin (ΔΔQTcF) exceeding 5 msec. Aficamten did not cause QTc prolongation (using C‐QT and by time point analyses) within observed plasma concentrations up to 1660 ng/mL (aficamten), 213 ng/mL (metabolite CK‐3834282), and 343 ng/mL (metabolite CK‐3834283). No clinically meaningful effect on electrocardiogram parameters, including absolute QTcF (≤ 450 msec) and change from baseline in QTcF (≤ 30 msec) was noted in aficamten‐treated participants. Aficamten was generally well tolerated. In conclusion, there was no evidence of aficamten‐mediated QTc prolongation across the therapeutic concentration range in a formal TQT study.