The least significant digit of a number written in a given positional base is the digit in the least significant place value, and has to be in the range . In the case of an integer, the least significant digit is the 1's place value, usually written to the right of the 's place value. In the case of a transcendental number, there is no actual least significant digit, but for computational purposes the rational approximation would have a least significant digit.

In an array of digits long meant for mathematical manipulation, it might be convenient to index the least significant digit with index 1 or 0, and the more significant digits with larger integers. (This enables the calculation of the value of a given digit as rather than .) For an array of digits meant for text string manipulation, however, the least significant digit might be placed at position (for example, by Mathematica's IntegerDigits function).

In binary, the least significant digit is often called the least significant bit.