Yahoo has agreed a deal with Google which will see Yahoo use the search engine giant's advertising technology.
Under the agreement, Google ads will appear alongside some Yahoo search results in the US and Canada.
The announcement came after Yahoo said it had failed to persuade Microsoft to renew its bid to buy all of the internet company.
Recent talks concluded after Yahoo rejected a Microsoft proposal to buy just its online search business.
Yahoo said the agreement with Google could be worth up to $800m (£410m) in additional revenue every year.
Alarm bells were already ringing on Capitol Hill over Yahoo's 'limited' trial in April of Google's technology
Rory Cellan-Jones, BBC Technology Correspondent
"This commercial agreement provides Yahoo with the opportunity to deliver more relevant ads to users and provide advertisers and publishers with better advertising technology," said Eric Schmidt, Google chairman and chief executive.
"We believe that the convergence of search and display is the next major development" in online advertising industry, said Yahoo chief executive Jerry Yang.
The partnership will initially last for three years, but could last up to 10 if Yahoo decides to renew.
Google said the deal did not need regulatory approval but that it would delay its implementation by up to three and a half months to give the US Department of Justice a chance to review it.
However, the deal is likely to attract attention from competition regulators in Washington, according to the BBC's technology correspondent Rory Cellan-Jones.
"Alarm bells were already ringing on Capitol Hill over Yahoo's 'limited' trial in April of Google's technology," he said.
The markets closed before the Google deal was announced. Shares in Yahoo had closed down 10% after it said its attempts to revive Microsoft's $47.5bn (£24bn) offer for the whole of Yahoo had been unsuccessful.
The shares fell as low as $22.50 earlier in the trading session, their lowest level since the software giant first offered $31 a share for Yahoo in January.
Yahoo rejected a new proposal from Microsoft to buy just its online search operations.
“In the weeks since Microsoft withdrew its offer to acquire Yahoo, the two companies have continued to discuss an alternative transaction that Microsoft believes would have delivered in excess of $33 per share to the Yahoo shareholders," Microsoft said in a statement.
"This partnership would ensure healthy competition in the marketplace, providing greater choice and innovation for advertisers, publishers and consumers," the company said.
Yahoo, however, did not want to sell off only one part of the business.
It said such a deal "would not be consistent with the company's view of the converging search and display marketplaces".
Microsoft sources told the BBC's Rory Cellan-Jones that the company was no longer interested in a full takeover because Yahoo had been "underperforming" and was losing some of its key staff.
Yahoo's shares closed 10% lower at $23.52, while Microsoft finished 4.1% higher at $28.24.