The Athletic, a subscription-based sports media startup, is preparing to more than double its staffing and expand to new professional sports markets after securing a new investment round.

Two years after launching as “the new sports page,” the Athletic has raised $20 million, according to Athletic co-founder and Chief Executive Alex Mather.

The funding round, the company’s third, was led by Evolution Media, the growth-stage investment company founded by TPG Growth and Creative Artists Agency. Before this round, the Athletic raised $10 million in two rounds led by Courtside Ventures.

Mr. Mather and his co-founder, Adam Hansmann, have positioned the Athletic as a reader-supported, advertising-free alternative to traditional sports coverage, at a time when nationally recognized outlets like ESPN and Sports Illustrated have cut positions and had to rethink their approach to sports news.

The Athletic plans to use most of the financing to continue its expansion across the U.S., establishing a presence in every market with a professional sports team by the end of the year. By the end of 2018, the Athletic plans to have between 200 and 350 employees, up from its current staff of 120. The company currently has a foothold in 23 markets across the U.S. and Canada, and plans to expand to roughly 45 markets by the end of the year.

In the works are city sites for Boston and Los Angeles and coverage of soccer in the U.S., Mexico and the U.K., Mr. Mather said. The company is also planning to establish teams dedicated to coverage of NFL and the NBA nationally.

The Athletic is among a group of digital media startups that has turned to subscriptions as a primary source of revenue at a time of profound disruption for the advertising industry. As Google and Facebook consume an ever-larger share of the digital advertising market, publishers such as the Athletic, tech news site the Information and the life sciences publication Stat are all betting that reader revenue is key to survival.

“What we’ve learned over the last couple of years is there’s an appetite for high-quality coverage, whether it’s local or national,” Mr. Mather said.

Mr. Mather said that the company’s subscribers have reached six figures, declining to be more specific. While the company isn’t yet profitable, Mr. Mather said that some of the company’s earliest markets—places like Chicago—have more than 25,000 subscribers and are profitable. The Athletic offers subscriptions for $8 a month or $48 a year.

The Athletic doesn’t have any advertising, but Mr. Mather said the company plans to begin testing sponsorship for events and podcasts in the coming months. He said the company is also considering syndication deals with local TV stations, but no such deals are in place yet.

News outlets that bank on subscription revenue are insulated from swings in the advertising market. But they face other challenges, such as the cost of retaining and acquiring readers and keeping subscribers interested after initial enthusiasm wanes in a new market.

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The Athletic says that roughly 90% of its subscribers renew annually, according to a company spokesperson, who added that the company is currently growing its oldest markets at a rate of about 20% month-over-month.

As many sports media outlets faced cutbacks in recent years, the Athletic is luring writers by offering a stake in the company and a break from the constant churn of blog posts that have come to define sports writing on the internet. The company is offering some writers premiums on salaries they earned at previous jobs and guaranteed contracts, according to a person familiar with the matter.

“Being a subscription site, it’s back to basics,” said Paul Fichtenbaum, chief content officer at the Athletic. “It’s back to storytelling.”

Other participating firms in the new funding round included Courtside Ventures, Luminari Capital, Advancit Capital, Bertelsmann Digital Media, the Chernin Group, LionTree Partners, Amasia, Y Combinator, Precursor Ventures, Chris Silbermann and Ali Rowghani.