As conventional electronics approaches its limits1, nanoscience has urgently sought methods of fast control of electrons at the fundamental quantum level2. Lightwave electronics3—the foundation of attosecond science4—uses the oscillating carrier wave of intense light pulses to control the translational motion of the electron’s charge faster than a single cycle of light5,6,7,8,9,10,11,12,13,14,15. Despite being particularly promising information carriers, the internal quantum attributes of spin16 and valley pseudospin17,18,21 have not been switchable on the subcycle scale. Here we demonstrate lightwave-driven changes of the valley pseudospin and introduce distinct signatures in the optical readout. Photogenerated electron–hole pairs in a monolayer of tungsten diselenide are accelerated and collided by a strong lightwave. The emergence of high-odd-order sidebands and anomalous changes in their polarization direction directly attest to the ultrafast pseudospin dynamics. Quantitative computations combining density functional theory with a non-perturbative quantum many-body approach assign the polarization of the sidebands to a lightwave-induced change of the valley pseudospin and confirm that the process is coherent and adiabatic. Our work opens the door to systematic valleytronic logic at optical clock rates.