Netflix Inc. said Monday its board adopted an antitakeover plan intended to block activist investor Carl Icahn from expanding his nearly 10% stake in the streaming video and DVD-by-mail company.

Under the plan, Netflix's shareholders will be given the right to acquire more stock if any individual investor acquires more than a 10% stake in the company, commonly referred to as a "poison pill." The plan is designed to dilute the value of the stock by flooding the market with additional shares, making it expensive for an investor...