A source familiar with the matter said the deal was worth about $1.1 billion.
KKR (KKR), which owns the popular Internet domain registration firm GoDaddy.com, is looking to expand Internet Brands' services while focusing on business categories such as automobiles, health, legal, and home and travel.
Internet Brands, which was taken private for $640 million in 2010, also operates websites such as Lawyers.com and ApartmentRatings.com. The company also provides automobile-related software and services to Toyota Motor (TM), Ford Motor (F) and Chrysler Group.
The company's websites receive more than 100 million visitors a month and most of its revenue comes from online advertising.
KKR said it was making the investment through its North America XI private equity fund.
The New York Times had earlier reported the deal.
KKR has no plans to break up the business, two people familiar with the matter told the New York Times.
Internet Brands' chief executive, Bob Brisco, and its management team will hold a minority stake and continue to run the company, the report said.
The El Segundo, California-based company has about 1,600 employees.
KKR acquired a majority stake in Norway-based accounting and payroll software maker Visma in 2010. The company recently cut its stake in Visma to 31.3 percent from 76 percent.
Shares of KKR closed at $22.90 on the New York Stock Exchange on Monday.