A stamp act is any legislation that requires a tax to be paid on the transfer of certain documents. Those who pay the tax receive an official stamp on their documents, making them legal documents. A variety of products have been covered by stamp acts including playing cards, dice, patent medicines, cheques, mortgages, contracts, marriage licenses and newspapers. The items may have to be physically stamped at approved government offices following payment of the duty, although methods involving annual payment of a fixed sum or purchase of adhesive stamps are more practical and common.

This system of taxation was first devised in the Netherlands in 1624 after a public competition to find a new form of tax. Stamp acts have been enforced in many countries, including Australia, Canada, People's Republic of China, Ireland, India, Malaysia, Israel, the United Kingdom, and the United States of America.

The taxes raised under a stamp act are called stamp duty.