In the sport of cricket, ball tampering is an action in which a fielder illegally alters the condition of the ball. The primary motivation of ball tampering is to interfere with the aerodynamics of the ball to aid swing bowling.

Under Law 41, subsection 3 of the Laws of Cricket, the ball may be polished without the use of an artificial substance, may be dried with a towel if it is wet, and have mud removed from it under supervision; all other actions which alter the condition of the ball are illegal. These are usually taken to include rubbing the ball on the ground, scuffing with a fingernail,using sandpaper or other sharp object, or tampering with the seam of the ball.

Generally, the purpose of altering the state of the ball is to achieve more favourable bowling conditions. Examples of ball tampering would include a fielder applying a substance, such as lip balm or sweetened saliva, to shine one side of the ball or pick the seam of the ball to encourage more swing. Conversely, roughening one side of the ball by use of an abrasive or cutting surface (such as boot spikes or bottle caps or sandpaper) is also ball tampering.

Using spit and/or sweat is common and, for practitioners of swing bowling, it was integral, though now considered illegal after the COVID-19 pandemic. The moisture gained from spit or sweat when combined with polishing, smooths out one half of the ball which in turn allows air to pass over one side of the ball more quickly than over the other. When bowled correctly, a bowler can get the ball to move from one side to the other through the air. Also, it is common for fielders to rub the ball against their clothing to dry or polish it, as seen in most cricket matches.

The umpires are responsible for monitoring the condition of the ball, and must inspect it regularly. Where an umpire has deemed a player to be guilty of ball tampering (the Laws refer to unfairly changing the condition of the ball), five penalty runs are awarded to the other side, and, if desired by the opposing captain, the ball is immediately replaced. The replacement ball is chosen by the umpires, and should match the condition of the previous ball (before tampering) as closely as possible. Depending on additional agreements laid out before the beginning of a series of matches, the team may instead be permitted to choose the ball from a selection of balls in various stages of use.

If a bowler is found to be guilty of repeated ball tampering he can be prohibited from continuing to bowl in that innings. Following the conclusion of play, additional sanctions are usually brought against a ball tamperer, as it is considered a serious offence. The captain may also be penalised, if he is also responsible for the conduct of his players on the field.

The use of foreign substances to polish the ball is illegal, but may be difficult to detect by the umpires. Saliva used to be commonly used to add shine to the ball. Due to the COVID-19 pandemic this has now been banned due to the risk of viral transmission through saliva. It is unclear if this rule will ever be reverted, and players are now using other legal ways to shine the ball such as sweat. Substances which have been used for this purpose include hair gel, sugar from sweets, and lip balm. Some commentators have suggested that this form of undetected ball tampering may be common.

Picking at the threads of the main seam or 'lifting' the quarter seam to aid conventional and reverse swing respectively are also illegal. Modifying the quarter seam can be particularly difficult to detect or prove.