MacKenzie Bezos has promised to give more than half of her fortune to charity as part of the Giving Pledge, the organization said Tuesday. Ms. Bezos, whose divorce from the Amazon founder and chief executive Jeff Bezos made news earlier this year, was among 19 new signatories announced by the group.

“I have no doubt that tremendous value comes when people act quickly on the impulse to give. No drive has more positive ripple effects than the desire to be of service,” Ms. Bezos, a novelist, wrote in a letter dated Friday.

Once the divorce is finalized in July, Ms. Bezos will hold a 4 percent stake in Amazon, with shares worth about $36 billion. Mr. Bezos kept most of the couple’s shares, or about 12 percent of the company. The value of the company on Tuesday is about $907 billion, meaning that Mr. Bezos, with a $108 billion stake, remains the richest person in the world, just ahead of Bill Gates.

Mr. Bezos, who has faced more public pressure to make large philanthropic investments as his wealth and influence have grown, has not joined the list of the wealthy who have committed to the Giving Pledge, started by Warren E. Buffett and Mr. Gates and Melinda E. Gates in 2010. It asks for a commitment by some of the world’s wealthiest people to donate much of their wealth to charity during their lifetimes or in their wills.