ValueAct owns less than 1 percent of the software company's outstanding shares, but is one of its largest shareholders.
Under an agreement announced Friday, ValueAct's President Mason Morfit will meet with Microsoft directors and management to discuss a range of business issues. The agreement also gives ValueAct the option of having Morfit join the Microsoft board of directors after the company's 2013 annual shareholders meeting.
"Microsoft is a world-class company with tremendous long-term potential," Morfit said in a statement. "At this critical inflection point in the company's evolution, I look forward to actively working together with the board and Microsoft's management team to continue to create value for all shareholders."
A representative for ValueAct could not be reached immediately for further details on the firm's plans.
Microsoft's CEO Steve Ballmer said the company is committed to enhancing growth for shareholders and looks forward to ValueAct's input.
The Redmond, Wash.-based company recently announced that Ballmer will retire sometime within the year. Investors are hoping the company can regain some of its edge under new leadership after struggling with a shift in consumer preferences away from personal computers and toward smartphones and tablets.
Shares of Microsoft added 13 cents in after-hours trading following the announcement to reach $33.53. Its shares lost 15 cents to close regular trading at $33.40 Friday, in line with a broader market dip.