Verizon doesn’t sell its mobile phones or wireless plans over Amazon. Nor does it offer Fios, its high-speed internet service. But Verizon does advertise on Amazon.

On Black Friday last year, when millions of online shoppers took to Amazon in search of deals, a Verizon ad for a Google Pixel 2 phone — buy one and get a second one half off — could be seen blazing across Amazon’s home page. And on July 16, what Amazon calls Prime Day, an event with special deals for its Prime customers, Verizon again ran a variety of ads and special offers for Amazon shoppers, like a mix-and-match unlimited service plan.

Amazon, which has already reshaped and dominated the online retail landscape, is quickly gathering momentum in a new, highly profitable arena: online advertising, where it is rapidly emerging as a major competitor to Google and Facebook.

The push by the giant online retailer means consumers — even Prime customers, who pay $119 a year for access to free shipping as well as streaming music, video and discounts — are likely to be confronted by ads in places where they didn’t exist before.