Pop-up ads or pop-ups are a form of online advertising on the WWW World Wide Web intended to attract web traffic or capture email addresses. It works when certain web sites open a new web browser window to display advertisements. The pop-up window containing an advertisement is usually generated by JavaScript, but can be generated by other means as well.

A variation on the pop-up window is the pop-under advertisement, which opens a new browser window hidden under the active window. Pop-unders do not interrupt the user immediately and are not seen until the covering window is closed, making it more difficult to determine which web site opened them.

Opera was the first major browser to incorporate tools to block pop-up ads; the Mozilla browser later improved on this by blocking only pop-ups generated as the page loads. In the early 2000s, all major web browsers except Internet Explorer allowed the user to block unwanted pop-ups almost completely. In 2004, Microsoft released Windows XP SP2, which added pop-up blocking to Internet Explorer.

Most modern browsers come with pop-up blocking tools; third-party tools tend to include other features such as ad filtering.

A combination of a banner ad and a popup window is the "hover ad", which uses DHTML to appear in front of the browser screen. With the use of JavaScript, an advertisement can be superimposed over a webpage in a transparent layer. This advertisement can appear as almost anything the author of the advertisement wants. For example, an advertisement can contain a Adobe Flash animation linking to the advertiser's site. An advertisement can also look like a regular window. Because the advertisement is a part of the web page, it cannot be blocked with third-party ad blockers such as Adblock or by using custom style sheets. DHTML ads can be very CPU intensive, sometimes bogging down older computers to the point of unusability.