A square has an area of 9 meters what is the side length in centimeters?

The area of a square is side * side, or side². Since the area is 9 square meters, the side length is the square root of 9, which is 3 meters.

To convert meters to centimeters, we multiply by 100 (since there are 100 centimeters in a meter).

Therefore, the side length is 3 meters * 100 centimeters/meter = $\boxed{300}$ centimeters.