The letter, posted on the American Association of Advertising Agencies and co-signed by 4A's president Nancy Hill, supports the two companies as they look to provide a real competitor to Google in the search sector.
Dated 19 October, the open letter calls on the Department of Justice to "bring its antitrust review to a speedy conclusion".
It reads: "A healthy, competitive market for search and search advertising is crucial to the Internet's future.
"We believe that Yahoo! and Microsoft's proposal to combine their technologies and search platforms is good for advertisers, marketing services agencies, website publishers and consumers."
Under the terms of the Yahoo!/Microsoft deal, the software giant's new engine Bing will power Yahoo!'s search results for the next 10 years, with Microsoft paying a proportion of the revenue gained from inserting advertising into the results back to Yahoo!.
The deal, which would give the two companies a combined share of about 30% of the US search market, was announced in July.
It ended a period of on-off talks which started as far back as January 2008 when Microsoft offered to buy Yahoo! outright for about $47bn.