It is a sharp escalation of its strike against Ford, General Motors and Stellantis, which began last month.
The UAW is pressuring the carmakers to grant higher pay and make other improvements in new labour agreements.
It is the first strike in the union's 88-year history to target the Detroit Three automakers, which collectively employ more than 140,000 UAW members, at the same time.
With Wednesday's walkout, which has halted production of lucrative pickup trucks, about a fifth of the auto workers have now downed tools.