As conventional electronics approaches its limits(1), nanoscience has urgently sought methods of fast control of electrons at the fundamental quantum level(2). Lightwave electronics(3)-the foundation of attosecond science(4)-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 light(5-15). Despite being particularly promising information carriers, the internal quantum attributes of spin(16) and valley pseudospin(17-21) 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.