A wash sale is a sale of a security (stocks, bonds, options) at a loss and repurchase of the same or substantially identical security (judging by CUSIP or Committee on Uniform Securities Identification Procedures numbers) shortly before or after. Losses from such sales are not deductible in most cases under the Internal Revenue Code in the United States. Wash sale regulations disallow an investor who holds an unrealized loss from accelerating a tax deduction into the current tax year, unless the investor is out of the position for some significant length of time. A wash sale can take place at any time during the year, or across year boundaries.

In the United Kingdom, a similar practice which specifically takes place at the end of a calendar year is known as bed and breakfasting. In a bed-and-breakfasting transaction, a position is sold on the last trading day of the year (typically late in the trading session) to establish a tax loss. The same position is then repurchased early on the first session of the new trading year, to restore the position (albeit at a lower cost basis). The term, therefore, derives its name from the late sale and early morning repurchase.

Wash sale rules don't apply when stock is sold at a profit. A related term, tax-loss harvesting is "selling an investment at a loss with the intention of ultimately repurchasing the same investment after the IRS's 30 day window on wash sales has expired". This allows investors to lower their tax amount with the use of investment losses. Wash sales and similar trading patterns are not themselves prohibited; the rules only deal with the tax treatment of capital losses and the accounting of the ongoing tax basis. Tax rules in the U.S. and U.K. defer the tax benefits of wash selling at a loss. Such losses are added to the basis of the newly acquired security, essentially deferring the tax benefits until a non-wash sale occurs, if ever.