San (Ϻ) was an archaic letter of the Greek alphabet. Its shape was similar to modern M, or to a modern Greek Sigma (Σ) turned sideways, and it was used as an alternative to Sigma to denote the sound /s/. Unlike Sigma, whose position in the alphabet is between Rho and Tau, San appeared between Pi and Qoppa in alphabetic order. In addition to denoting this separate archaic character, the name "San" was also used as an alternative name to denote the standard letter Sigma.

The existence of the two competing letters Sigma and San is believed to have been due to confusion during the adoption of the Greek alphabet from the Phoenician script, because Phoenician had more sibilant (s-like) sounds than Greek had. According to one theory, the distribution of the sibilant letters in Greek is due to pair-wise confusion between the sounds and alphabet positions of the four Phoenician sibilant signs: Greek Sigma got its shape and alphabetic position from Phoenician Šin (), but its name and sound value from Phoenician Samekh. Conversely, Greek Xi (Ξ) got its shape and position from Samekh (), but its name and sound value from Šin. The same kind of pair-wise exchange happened between Phoenician Zayin and Tsade: Greek Zeta has the shape and position of Zayin () but the name and sound value of Tsade, and conversely Greek San has the approximate shape and position of Tsade () but may originally have had the sound value of Zayin, i.e. voiced [z]. However, since voiced [z] and voiceless [s] were not distinct phonemes in Greek, Sigma and San came to be used in essentially the same function.