Cost per action (CPA), also sometimes misconstrued in marketing environments as cost per acquisition, is an online advertising measurement and pricing model referring to a specified action, for example, a sale, click, or form submit (e.g., contact request, newsletter sign up, registration, etc.).

Direct response advertisers often consider CPA the optimal way to buy online advertising, as an advertiser only considers the measured CPA goal as the important outcome of their activity The desired action to be performed is determined by the advertiser. In affiliate marketing, this means that advertisers only pay the affiliates for leads that result in the desired action such as a sale. This removes the risk for the advertiser because they know in advance that they will not have to pay for bad referrals, and it encourages the affiliate to send good referrals.

Radio and TV stations also sometimes offer unsold inventory on a cost per action basis, but this form of advertising is most often referred to as "per inquiry". Although less common, print media will also sometimes be sold on a CPA basis.