Reliable modulation of terahertz electromagnetic waveforms is important for many applications. Here, we rapidly modulate the direction of the electric field of linearly polarized terahertz electromagnetic pulses with 1–30 THz bandwidth by applying time-dependent magnetic fields to a spintronic terahertz emitter. Polarity modulation of the terahertz field with more than 99% contrast at a rate of 10 kHz is achieved using a harmonic magnetic field. By adding a static magnetic field, we modulate the direction of the terahertz field between angles of, for instance, −53° and 53° at kilohertz rates. We believe our approach makes spintronic terahertz emitters a promising source for low-noise modulation spectroscopy and polarization-sensitive techniques such as ellipsometry at 1–30 THz.