The so-called poison pill is triggered if a person or group acquires beneficial ownership of 20% or more of Navient common stock.
"In adopting the Rights Plan, the board noted, in particular, the recent stock activity and the accumulation of a substantial economic position in the company by entities associated with Sherborne Investors Management LP," Navient said.
“Navient is committed to engaging in constructive dialogue with all of our investors and we welcome their perspectives,” Jack Remondi, president and chief executive, said in a statement.
“We also want to ensure investors are able to realize the full long-term value of their investment and receive fair and equal treatment, which is what the rights plan is designed to do.”
The Wilmington, Del., company's move comes as one of the funds owned by Sherbone Investors, New York. The firm, led by activist investor Edward Bramson, has built a 16% stake in Navient.
That stake makes Sherborne Navient's biggest investor. Vanguard Group has a 10.66% stake, according to Reuters.
The rights plan has several "recognized shareholder-friendly features," according to Navient, including a one-year term and an exception for fully financed, all-cash offers that are open for at least 60 business days.
A week ago Navient said the board had approved a share buyback of as much as $1 billion, in addition to the $150 million remaining on a previous authorization.
Navient shares at last check were trading off 5.8% at $20.06.