The Halbach array, originally developed for particle accelerators, is a compact arrangement of permanent magnets that creates well-defined magnetic fields without heating. Here, we demonstrate its use for modulating the speed of electromechanical waves in cardiac syncytia of human stem cell-derived cardiomyocytes. At 40-50 mT magnetic field strength, a cylindrical dipolar Halbach array boosted the conduction velocity (CV) by up to 25% when the magnetic field was co-aligned with the electromechanical wave (but not when perpendicular to it). To observe the effects, a short-term incubation of the cardiac cell constructs with non-targeted magnetic nanoparticles (mNPs) was sufficient. This led to increased CV anisotropy, and effects were most pronounced at slower pacing rates. Instantaneous formation and rearrangement of elongated mNP clusters upon magnetic-field rotation was seen, creating dynamic structural anisotropy that may have contributed to the directional CV effects. This approach may be useful for anti-arrhythmic control of cardiac waves.