Online advertising, also known as online marketing, Internet advertising, digital advertising or web advertising, is a form of marketing and advertising that uses the Internet to promote products and services to audiences and platform users. Online advertising includes email marketing, search engine marketing (SEM), social media marketing, many types of display advertising (including web banner advertising), and mobile advertising. Advertisements are increasingly being delivered via automated software systems operating across multiple websites, media services and platforms, known as programmatic advertising.

Like other advertising media, online advertising frequently involves a publisher, who integrates advertisements into its online content, and an advertiser, who provides the advertisements to be displayed on the publisher's content. Other potential participants include advertising agencies that help generate and place the ad copy, an ad server which technologically delivers the ad and tracks statistics, and advertising affiliates who do independent promotional work for the advertiser.

In 2016, Internet advertising revenues in the United States surpassed those of cable television and broadcast television. In 2017, Internet advertising revenues in the United States totaled $83.0 billion, a 14% increase over the $72.50 billion in revenues in 2016. And research estimates for 2019's online advertising spend put it at $125.2 billion in the United States, some $54.8 billion higher than the spend on television ($70.4 billion).

Many common online advertising practices are controversial and, as a result, have become increasingly subject to regulation. Many internet users also find online advertising disruptive and have increasingly turned to ad blocking for a variety of reasons. Online ad revenues also may not adequately replace other publishers' revenue streams. Declining ad revenue has led some publishers to place their content behind paywalls.

In the early days of the Internet, online advertising was mostly prohibited. For example, two of the predecessor networks to the Internet, ARPANET and NSFNet, had "acceptable use policies" that banned network "use for commercial activities by for-profit institutions". The NSFNet began phasing out its commercial use ban in 1991.

The first widely publicized example of online advertising was conducted via electronic mail. On 3 May 1978, a marketer from DEC (Digital Equipment Corporation), Gary Thuerk, sent an email to most of the ARPANET's American West Coast users, advertising an open house for a new model of a DEC computer. Despite the prevailing acceptable use policies, electronic mail marketing rapidly expanded and eventually became known as "spam."

The first known large-scale non-commercial spam message was sent on 18 January 1994 by an Andrews University system administrator, by cross-posting a religious message to all USENET newsgroups. In January 1994 Mark Eberra started the first email marketing company for opt-in email lists under the domain Insideconnect.com. He also started the Direct Email Marketing Association to help stop unwanted email and prevent spam.

Four months later, Laurence Canter and Martha Siegel, partners in a law firm, broadly promoted their legal services in a USENET posting titled "Green Card Lottery – Final One?" Canter and Siegel's Green Card USENET spam raised the profile of online advertising, stimulating widespread interest in advertising via both Usenet and traditional email. More recently, spam has evolved into a more industrial operation, where spammers use armies of virus-infected computers (botnets) to send spam remotely.