Yocto- (symbol y) is a unit prefix in the metric system denoting a factor of 10 or 0.000000000000000000000001.

It was adopted in 1991 by the General Conference on Weights and Measures. It comes from the Latin/Greek octo (οκτώ), meaning "eight", because it is equal to 1000. Yocto- is the smallest official SI prefix.