According to Cheddar, which cites people familiar with the talks, the deal is closed to being finalised and while a price hasn't been confirmed, one source suggested that it would be at least $2bn (£1.52bn).
AppNexus, which recently launched a new product offering 100 per cent viewability guaranteed, has consistently described itself as "the world's largest independent marketplace for digital advertising". This deal could potentially end that, but would also see AT&T establish itself as a potential challenger to Google and Facebook when it comes to online advertising.
While neither AT&T nor AppNexus have commented on reports of the acquisition, Randal Stephenson, CEO of AT&T, told CNBC last week that the company was in the process of establishing a "significant advertising platform" and was planning to announce at least on other acquisition "in the coming weeks to demonstrate our commitment to that."
AT&T's current advertising business is led by Brian Lesser, former CEO of GroupM North America, who joined the telco in October 2017. Lesser also previously sat on the board of AppNexus, and AT&T is already a significant customer of the ad tech firm.
Rumours about an AppNexus IPO have often circulated in the past, with the firm reportedly considering an initial offering in early 2017 but backing out of the process. The New York-based firm has raised roughly $344m in private funding since it was founded in 2007, and is "very, very financially healthy", according to CEO Brian O'Kelly.