Yes, the square root of 81 is a rational number. A rational number is any number that can be expressed as the quotient or fraction p/q of two integers, where p is the numerator and q is the non-zero denominator.
The square root of 81 is 9. Since 9 can be expressed as 9/1, where both 9 and 1 are integers, it fits the definition of a rational number. Therefore, the square root of 81 is indeed a rational number.