The media’s “Trump Bump” is starting to get the attention of investors.

The Young Turks Network, the left-leaning, YouTube-driven political news outlet led by outspoken activist Cenk Uygur, has raised $20 million, which it plans to use to double the size of its newsroom and business operations.

The funding round is being led by newly formed growth equity fund 3L Capital, with additional backing from venture capital veterans Greycroft Partners and e.ventures, as well as Jeffrey Katzenberg’s recently launched WndrCo.

“News is red hot right now and being positioned on the progressive side of the coin has put us in a very good place,” Mr. Uygur said.

He said the company currently has about 70 staffers and 30 freelancers but plans to use the investment to increase its staff to about 200 people. Part of that expansion will be in creative talent, but Mr. Uygur said it would also entail building an ad sales team and bolstering its paid subscription offerings.

The valuation of Young Turks in the deal wasn’t disclosed.

Launched on YouTube in 2005, Young Turks’ flagship political commentary channel has grown to 3.4 million subscribers and serves as the anchor for a raft of other, smaller channels focused on sports, film and tech.

Young Turks -- which threw it support behind the candidacy of Bernie Sanders and has been sharply critical of Donald Trump’s presidency -- has enjoyed strong growth over the past year across many of its channels.

Its primary channel saw 14% growth in subscribers and a 30% increase in views on YouTube compared to July 2016, according to OpenSlate.

The company has largely grown without the benefit of major investment. In 2014, it signed a $4 million convertible note with an investment firm led by former Louisiana governor Buddy Roemer. The following year, it took in $4.25 million from an undisclosed group of “non-institutional” investors who Mr. Uygur described as “smart, savvy people in the industry.” And in May, it raised $2 million through a crowdfunding campaign in order to hire a dozen investigative journalists.

Versions of Young Turks content have appeared on cable TV and satellite radio over the years. Mr. Uygur has also targeted over-the-top streaming services, which the company’s investors see as fertile territory for growth. The company is currently available on Pluto TV and Comcast’s “Watchable.”

“The macro trends around how millennials are consuming news and their political interest has made this a good business opportunity for us,” said Shawn Colo, a partner at 3L Capital.

Corrections & Amplifications

Data on the Young Turks’ audience growth on YouTube was provided by OpenSlate. An earlier version of this story incorrectly referred to the company as Open Slate Data.

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