A Set Number (known as Card Number in Japanese OCG, カードナンバー, Kādonanbā; not to be confused with Card Number in the TCG) is the ID code found immediately underneath the image on most OCG and TCG cards, on the right-hand side (opposite the edition text, when that text appears). The general format for set numbers is "SET-LN999", where "SET" is the two-, three-, or four-character set abbreviation, "LN" is the one- or two-letter language/region abbreviation, and "999" is the set card number. Some older cards, depending on the region, did not have a language abbreviation in their set number, and very old OCG cards did not have a set number at all.

A very limited number of cards have nonstandard set numbers, such as SD1-ENDE1 (Armed Dragon LV7), which, instead of the usual three-digit set card number, has "DE" and a one-digit number.

See OCG Set Prefixes and TCG Set Prefixes for more information on set abbreviations, and Card Number for the eight-digit number in a card's lower left-hand corner.