Google said it expected to sell shares mainly to index funds who will have to own a stake in the firm when it joins the S&P 500.
If sold at Wednesday's closing price of $394.98, Google could raise as much as $2.1bn through its sale of extra shares.
However, shareholders fear Google's plan could mean the company will miss high earnings per share targets set by analysts. As a result, the group's shares slipped in after-hours trade.
Official documents it filed with US market watchdogs gave little information about why the group wanted to raise the extra cash.
Advertising expansion
The plans for the share sale were revealed as Google finalised the details of its tie-up with AOL, which was announced late last year.
Under the deal, Google will take a 5% stake in its biggest advertising partner in return for a $1bn investment to be made some time in the three months to the end of June.
AOL will also receive a $300m credit to advertise its products and services on Google, in return AOL is expected to sell more graphical adverts.
So far Google has mainly relied on adverts made up of a few words of text and a link.
In 2005, AOL accounted for about $550m, or 9%, of Google's total revenue.