In baseball, a pitcher can commit a number of illegal motions or actions that constitute a balk. Most of these violations involve pitchers pretending to pitch when they have no intention of doing so. In games played under the Official Baseball Rules that govern professional play in the United States and Canada, a balk results in a dead ball or delayed dead ball. In certain other circumstances, a balk may be wholly or partially disregarded. Under other rule sets, notably in the United States under the National Federation of State High School Associations (NFHS Baseball Rules), a balk results in an immediate dead ball. In the event a balk is enforced, the pitch is generally (but not always) nullified, each runner is awarded one base, and the batter (generally) remains at bat, and with the previous count. The balk rule in Major League Baseball was introduced in 1898.

The reason a balk is prohibited is given in the Official Baseball Rules: "Umpires should bear in mind that the purpose of the balk rule is to prevent the pitcher from deliberately deceiving the base runner."