What is the square root of 81?

The square root of 81 is 9. This is because 9 multiplied by itself (9 x 9) equals 81. It's also worth noting that -9 is also a square root of 81, since (-9) x (-9) = 81. However, when people refer to "the" square root, they usually mean the principal square root, which is the positive one (9).