WASHINGTON — Adam Mosseri, the head of Instagram, is appearing before a Senate panel on Wednesday afternoon to defend the social media app from growing bipartisan outrage over its reported harms to young users.

It is Mr. Mosseri’s first appearance before Congress. He is the highest-ranking official from Meta, the company formerly known as Facebook and the parent company of Instagram, to testify to lawmakers after a whistle-blower leaked internal research that said Instagram had a toxic effect on some teenagers.

“Facebook’s own researchers have been warning management, including yourself, Mr. Mosseri, for years,” Richard Blumenthal, Democrat of Connecticut and chair of the Senate Commerce Committee’s panel on consumer protection, said at the start of the hearing. “Parents are asking, what is Congress doing to protect our kids and the resounding bipartisan message from this committee is that legislation is coming. We can’t rely on self-policing.”

Lawmakers are expected to grill Mr. Mosseri about the research, which showed that a third of teenage girls said the app made them feel worse about their body image. He will probably also be questioned about the app’s underlying technology and whether it sends young users into rabbit holes of more dangerous and harmful content. Republican and Democratic lawmakers say they will also confront him about the safety of young users, including the company’s efforts to keep underage users off the site.

At the beginning of the hearing, Mr. Mosseri said Instagram has a positive role in the lives of teenagers, such as by helping young users establish connections during difficult times. He also recognized the skepticism among members of Congress toward Meta.

“I recognize that many in this room have deep reservations about our company,” Mr. Mosseri said, “but I want to assure you that we do have the same goal. We all want teens to be safe online.”

On Tuesday, Instagram announced new safety features for children. Mr. Mosseri noted those changes, which include tools like a “take a break” feature that is meant to help limit time spent online. (TikTok has a similar function that appears when users are spending too much time on the app.)

Mr. Mosseri, 38, is a longtime executive at Facebook and considered a close lieutenant of the company’s chief executive, Mark Zuckerberg. He joined the company in 2008 as a designer and gradually rose in the ranks to run the News Feed, a central feature of the Facebook app.

In October 2018, he was named head of Instagram, weeks after the sudden resignations of the app’s founders, Kevin Systrom and Mike Krieger.

This is the fifth hearing by the Senate consumer protection subcommittee on protecting children online, and executives of TikTok and YouTube have already appeared. But Instagram became the focus of lawmakers after a former Facebook employee, Frances Haugen, leaked internal research that showed some troubling findings about the toxic role Instagram plays in the lives of young users, particularly teenage girls.

Richard Blumenthal, the chair of the subcommittee and a Democrat of Connecticut, has said his office had received hundreds of calls and emails from parents about their negative experiences with Instagram. One parent recounted how her daughter’s interest in fitness on Instagram led the app to recommend accounts on extreme dieting, eating disorders and self-harm.

Mr. Blumenthal has homed in on the algorithms that push such recommendations.

Lawmakers, including Mr. Blumenthal and Ms. Blackburn, have proposed stronger data privacy rules aimed at protecting children, greater enforcement of age restrictions and the ability of young users to delete information online.