The landmark ad site is the height of a seven-storey building and is situated on the M4 flyover on the western approach to London.
Foster & Partners has built the structure, called the JCDecaux M4 Tower, from glass. It consists of two 50m-square backlit panels that face in both directions and incorporate thousands of LEDs.
The Dubai Development and Investment Authority is the site's first advertiser.
It has signed up for a year-long campaign, which was booked by Manning Gottlieb OMD through the outdoor specialist Posterscope.
Jeremy Male, JCDecaux's UK and Northern Europe chief executive, said: "The M4 Tower sets a new benchmark in outdoor advertising and underpins our commitment to high-quality structures."
The M4 carries 1,630,000 people a week, according to figures from the Department of the Environment, Transport and the Regions.