SAN FRANCISCO — Jack Dorsey is stepping down as chief executive of Twitter, the social media site he co-founded in 2006 and guided through the tumultuous years of the Trump administration.

Twitter announced Mr. Dorsey’s departure on Monday. He is being replaced by Parag Agrawal, the company’s current chief technology officer. Mr. Dorsey’s plans were first reported by CNBC. A Twitter spokeswoman did not respond to a request for comment.

Mr. Dorsey’s exit will mark a significant shift at the company, which has navigated years of pressure from investors and increasing criticism from Washington, particularly Republican lawmakers who complain Twitter has contributed to a stifling of conservative voices in social media.

The most prominent of those voices was that of former President Donald J. Trump, who used his Twitter feed to threaten enemies and keep his allies in line. Twitter banned Mr. Trump shortly after the Jan. 6 attack on the Capitol.

Mr. Dorsey, who is also the chief executive of the payments company Square, was fired from the top job at Twitter in 2008 but returned in 2015.

His leadership has been questioned by employees and investors who believed that he was unfocused and spent too much of his time on Square and other passion projects. His departure comes a year and a half after Mr. Dorsey survived an attempted ouster from the activist investor Elliott Management.

Chief among Elliott’s concerns was that Mr. Dorsey’s attention was divided between the two companies he led. The firm believed that Twitter had fallen behind social media rivals in increasing its stock price and adding innovative new products.

Some employees rallied around Mr. Dorsey during the attempt to force him out, using the hashtag #WeBackJack as a rallying cry.

In March 2020, Elliott Management struck a deal with Silver Lake, one of Silicon Valley’s biggest investors in technology companies, that allowed Mr. Dorsey to stay on at Twitter. The deal also gave Jesse Cohn, the Elliott executive who oversaw the Twitter campaign, a seat on Twitter’s board, which he relinquished in June.

After the takeover attempt, Twitter’s stock began to climb, and in February, Mr. Dorsey announced an ambitious plan to double Twitter’s revenue by the end of 2023.

But some of Twitter’s stock market gains have slipped away in recent months, with the stock now worth roughly the same as it was a year ago. In the third quarter, Twitter said its revenue grew 37 percent from a year ago, to $1.28 billion, but that it incurred a loss of $537 million.

In addition to investor scrutiny, Mr. Dorsey has also faced pressure from legislators. Some have demanded that the company do more to address misinformation and hate speech on the platform, while others have accused Mr. Dorsey of censorship and argued Twitter should allow more content to remain online.

The moderation issues, and the legislative debate swirling around them, have been a persistent irritation for Mr. Dorsey. He had envisioned Twitter as a platform for free speech and bristled at the idea of removing content, especially from world leaders and other newsworthy figures.

But Mr. Trump’s incendiary tweets tested Mr. Dorsey’s stance. Twitter at first compromised by labeling some of Mr. Trump’s tweets as misinformation, before finally removing his account.

The majority of Mr. Dorsey’s wealth comes from Square, which he founded in 2009 during his last departure from Twitter. Last April, Mr. Dorsey announced he would donate $1 billion, or nearly a third of his total wealth, to relief programs related to the coronavirus and other philanthropic endeavors.

Twitter’s stock jumped 5 percent on the news before a halt in trading.

On Sunday night, in what was perhaps a foreshadowing of the news to come, Mr. Dorsey tweeted, “I love Twitter.”

In an email to Twitter employees announcing his departure, Mr. Dorsey said he wanted Twitter to stop being a founder-led company, which could be a weakness over time. “I’ve worked hard to ensure this company can break away from its founding and founders,” he wrote. “I believe it’s critical a company can stand on its own, free of its founder’s influence or direction.”

“There aren’t many companies that get to this level. And there aren’t many founders that choose their company over their own ego,” Mr. Dorsey added.

This is a developing story. Check back for updates.