A knuckleball or knuckler is a baseball pitch thrown to minimize the spin of the ball in flight, causing an erratic, unpredictable motion. The air flow over a seam of the ball causes the ball to change from laminar to turbulent flow. This change adds a deflecting force to the baseball, making it difficult for batters to hit but also difficult for pitchers to control and catchers to catch; umpires are challenged as well, as the ball's irregular motion through the air makes it harder to call balls and strikes. A pitcher who throws knuckleballs is known as a knuckleballer.

In baseball, the pitch is the act of throwing the baseball toward home plate to start a play. The term comes from the Knickerbocker Rules. Originally, the ball had to be thrown underhand, much like "pitching in horseshoes". Overhand pitching was not allowed in baseball until 1884.