The Jack is a playing card with a picture of a young man on it. The usual rank of a Jack is as if it were an 11 (that is, below the Queen and above the 10.) However, Jacks are the highest possible cards in the German game Skat, and are lower than 10s in many other games, like Pinochle.

As the lowest face (or "court") card, the Jack represents a minimum standard. Many poker games require a minimum hand of a pair of Jacks, often called simply "Jacks or better".

As early as the mid-1500s, the Jack was called the Knave. The card came to be known as the Jack during the middle of the 19th century, when card manufacturers began to label playing cards to indicate their value, i.e. 2, 3, 4, 5, 6, 7, 8, 9, 10, Kn, Q, K, A. The obvious confusion between Kn and K led to the renaming of the Knave, being out-ranked by the King. However, books of card games published in the third quarter of the 19th century evidently still referred to the "Knave."

In the standard English playing card deck, the Jack and the other face cards represent no one in particular. However, the court cards in a French deck do have names. Confusion often arises because the suits in an English deck came from the French deck as well.

Following are the personages represented by the Jacks in the French deck:

• Jack of Spades: Ogier the Dane (fictional hero of the chansons de geste)
• Jack of Hearts: La Hire (French warrior)
• Jack of Diamonds: Hector (mythological hero of the Iliad)
• Jack of Clubs: Lancelot (fictional hero of the Arthurian legend)