Arianna Huffington, who co-founded the Huffington Post 11 years ago and built it into one of the largest digital media outlets in the U.S., says she will be leaving the company in the coming weeks to focus on a soon-to-launch startup dedicated to health and wellness.

Ms. Huffington, who is 66 years old, had signed a new deal in June 2015 to remain at the Huffington Post as president and editor in chief through 2019. But she said the contract allowed her to launch the new company, Thrive Global, while continuing in her role at the site.

When she secured Series A funding for the new venture late last week, she said it had become clear she could no longer continue running Huffington Post. “I really thought I could do both, but as we started building it up, I realized that it really needed my full attention,” she said in an interview. “It is important to know when one door closes and another opens, and I felt that moment had arrived.”

Ms. Huffington’s continued role at her namesake site—which was acquired by AOL Inc. in 2011 for $315 million—had become unclear after AOL was bought by Verizon Communications Inc. for $4.4 billion last year. The matter appeared put to rest when she signed the new contract, but her future with the site came into question again when she announced two months ago that she would be launching a new venture.

Ms. Huffington has published a number of books focusing on health and sleep issues, including “Thrive: The Third Metric to Redefining Success and Creating a Life of Well-Being, Wisdom, and Wonder,” and “The Sleep Revolution.” She had spent an increasing amount of time this year dedicated to giving lectures on the subjects and less time running the site.

Ms. Huffington, a Greek native, rose to U.S. prominence in the 1990s as a conservative political commentator. Her then-husband, Michael Huffington, a billionaire Republican oil heir, ran for the U.S. Senate in California in 1994 but narrowly lost to Dianne Feinstein. After they divorced, Ms. Huffington became more liberal and in 2005 co-founded the Huffington Post along with Kenneth Lerer and Jonah Peretti.

Kenneth Lerer and Arianna Huffington at a Huffington Post event in 2010. Photo: Getty Images

The site helped pioneer digital-only journalism and its success would serve as a template for many digital startups that followed in its wake.

Mr. Peretti later founded BuzzFeed with the backing of Mr. Lerer who has become a top venture capitalist for many digital-media companies and is investing in Ms. Huffington’s new venture.

“When we started, there was no Facebook, there was no video, no social. It was tech 101, but Arianna saw the possibilities,” said Mr. Lerer. “Without her marketing genius, her editing and her contacts, none of this would have happened.”

When the Huffington Post was sold to AOL five years ago, Ms. Huffington was named president and editor in chief of Huffington Post Media Group, which took on other AOL media properties such as Engadget, TechCrunch, Moviefone and MapQuest.

The management structure, however, created tension, and by mid-2012, her portfolio was scaled back to only the Huffington Post.

In recent years, Huffington Post has undergone an aggressive global expansion, launching localized versions of the site in 14 countries, such as Greece, the U.K., France and Mexico.

The Huffington Post was founded as a liberal alternative to the Drudge Report and quickly rose in prominence as a blogging platform and news aggregator. Ms. Huffington often leveraged her personal relationships to get celebrities and political figures to write columns for the site.

“Arianna is a visionary who built The Huffington Post into a truly transformative news platform,” Tim Armstrong, chief executive officer of AOL, said in a statement. “AOL and Verizon are committed to continuing its growth and the groundbreaking work Arianna pioneered.”

Arianna Huffington, co-founder and editor-in-chief of the Huffington Post, is leaving the digital media outlet to focus on her new startup Thrive Global. Photo: Getty Images for AOL

Ms. Huffington’s time running the company wasn’t without controversy.

Huffington Post’s practice of not paying many of its contributors—arguing that they were benefiting from exposure to the site’s large number of visitors—brought criticism. In 2011, the National Writers Union and the News Guild called for a boycott of the site.

Despite regularly drawing more than 100 million unique visitors a month, the site has failed to consistently turn a profit.

In April, Ms. Huffington raised concerns among staff when she joined the board of Uber, but she said she recused herself from coverage of the ride-sharing business.

Thrive Global, which will work with companies to improve the well-being of their employees, is slated to formally launch in November. The company’s Series A funding round was led by Lerer Hippeau Ventures, Mr. Lerer’s fund.

In a memo, Mr. Armstrong said an interim editorial committee will fill Ms. Huffington’s role while helping search for a new editor in chief. Ms. Huffington said there had been discussions for her to continue in some role, but she instead decided to leave in the next two or three weeks.

“I have been running Huffington Post for 11 years and I couldn’t imagine having another relationship with it,” she said.

Corrections & Amplifications:

The Huffington Post was co-founded by Arianna Huffington, Kenneth Lerer and Jonah Peretti. An earlier version of this story incorrectly stated that Andrew Breitbart was also a co-founder.

Write to Lukas I. Alpert at lukas.alpert@wsj.com