For much of the last nine years, Zillow and Trulia have competed in the online real estate listings market they helped create.

But after a speedy six-week courtship, the two are set to combine forces.

Zillow agreed Monday to buy Trulia for about $3.5 billion in stock, creating a giant online repository of real estate listings and home values.

Under the terms of the deal, Zillow will pay 0.444 of one of its shares for each share of Trulia. Based on Friday’s closing prices, the takeover bid is worth $70.53 a Trulia share, a premium of roughly 25 percent.

Together, the two will dominate the traffic for online home listings. Last month, Zillow reported 83 million users, while Trulia reported 54 million — a combined 61 percent of total Internet users for the category, according to the research firm comScore.

“The companies know each other very well,” Spencer Rascoff, Zillow’s chief executive, said in a telephone interview. “We’ve been competitors and rivals for nine years, but I’ve always had respect for them.”

Zillow has already become one of the best-known players in the market through its widely quoted “Zestimates” of how much a property is worth, a feature particularly popular among homeowners. Trulia, on the other hand, has tools that tend to draw more potential home sellers and buyers.

According to Rascoff, that has meant relatively little overlap, with about half of Trulia’s web users not visiting Zillow.

Rascoff said he first approached Trulia about a potential acquisition about six weeks ago.

“Both companies are coming at this from a lot of strength and momentum,” Rascoff said. “When we approached them, I think they were both very open-minded about it.”

Trulia’s management team, led by Pete Flint, ultimately proved willing to negotiate but requested that it be an all-stock deal to give Trulia’s shareholders a chance to benefit from the merger.

Existing Trulia shareholders will own about a third of the combined company. Flint will stay on and report to Rascoff, and he and another Trulia director will join Zillow’s board.

Together, the two companies expect to realize about $100 million in cost savings by 2016. Rascoff said he did not expect the merger to face opposition from antitrust regulators.