<p>Today’s information processing technology relies on electronics, with transistor switches reaching speeds as high as 800 GHz and their intrinsic limit being set by charge-carrier transit times. The next step towards increasing the speed of information processing could come from driving the electronic response in solids using ultrafast controlled lightwaves. Such lightwave electronics aims to use ultrashort pulses of light to switch electric currents and can operate at near-petahertz rates. Lightwave valleytronics targets the valley pseudospin degree of freedom for information processing offered by two-dimensional materials. Here we use a sequence of phase-locked few-optical-cycle visible pulses to excite and switch the valley pseudospin in a WS₂ monolayer. By timing the carrier oscillations with subfemtosecond precision, we show that a pair of pulses separated in time with linear orthogonal polarizations can induce a valley-selective population. Adding a second pair of pulses, we perform logic operations such as valley de-excitation and re-excitation at room temperature at rates as high as 10 THz. Our experimental method enables independent measurements of the valley polarization decay and the excitonic decoherence time, opening a route to ultrafast information processing with low-power few-optical-cycle light pulses that are already available.</p>