Twitter is taking countermeasures to resist a takeover by tech billionaire Elon Musk.
The plan provides that other shareholders can buy additional shares cheaply if a buyer exceeds the 15 percent mark.
The step is considered a classic "poison pill" that companies threatened by takeovers often resort to.
The measure should apply for one year, the board of directors announced on Friday.
It is also planned that in the event of a possible takeover of the company through the large-scale purchase of shares, the buyer will have to pay a so-called control premium to the other shareholders.
This is a contribution that is above the market value of the shares.
The approved plan will reduce the likelihood that "any institution, person or group" will gain control of the company by acquiring shares in the open market without paying all shareholders a "reasonable control premium," Twitter said.
Musk, founder of electric car maker Tesla and space company SpaceX, announced Thursday an offer to all Twitter shareholders to buy shares at a price of $54.20.
In the past few weeks, he has already bought a stake of 9.2 percent.
Musk put the value of Twitter at around $43 billion.
However, the star entrepreneur admitted on Thursday that he was "not sure" that he would succeed in taking over Twitter.
Investors on the stock exchange do not believe that Musk will reach his goal either: in New York trading, the share price fell by around 1.7 percent to the $ 45 mark in the middle of the day.