AT&T Inc.’s advertising chief said the company’s acquisition of AppNexus is part of a strategy to build a first-of-its-kind marketplace for television and digital video advertising and give it more firepower against industry juggernauts Google and Facebook Inc.

Terms of the AppNexus deal, which was announced Monday, weren’t disclosed. The Wall Street Journal reported last week AT&T was expected to pay around $1.6 billion, according to people familiar with the matter.

AppNexus...