The Grand Canyon was not discovered by one single person. Native peoples had lived in and around the canyon for thousands of years before Europeans arrived, so the idea of a lone “discoverer” is misleading.
If the question means the first European to reach or describe it, the usual name is García López de Cárdenas, a Spanish explorer who visited the canyon area in 1540.
If the question means who first made the canyon widely known to Americans through scientific exploration, that name is John Wesley Powell, who led the famous river expedition in 1869 and helped reveal the canyon’s scale and geography to a broader audience.
So the shortest accurate answer is: no one discovered the Grand Canyon in the absolute sense, but García López de Cárdenas is often credited as the first European known to have seen it, and John Wesley Powell made it famous later through exploration and study.