We map agricultural worker exposure to unsafe heat stress using high-resolution hourly climate projections and gridded workforce data. By 2050, globally workers facing over 100 days with heat stress each year will nearly double from 359 to 663 million. Under moderate emissions, 8% of standard workdays will be lost to unsafe conditions, rising to 11% under high emissions. In tropical regions, extreme heat will encroach evening hours, limiting adaptation strategies and underscoring urgent need for climate-resilient worker protections.