A spendthrift (also profligate or prodigal) is someone who spends money prodigiously and who is extravagant and recklessly wasteful, often to a point where the spending climbs well beyond his or her means. "Spendthrift" derives from an obsolete sense of the word "thrift" to mean prosperity rather than frugality, so a "spendthrift" is one who has spent their prosperity.

Historical figures who have been characterized as spendthrifts include Karl Marx, George IV of Great Britain, King Ludwig II of Bavaria, and Marie Antoinette the Queen of France.

The term is often used by the press as an adjective applied to governments who are thought to be wasting public money.

While the pair of words may seem to imply the opposite of its meaning (as if you are thrifty in your spending), it follows the tradition of the earlier word "scattergood", the first part being an undoing of the second.