Jack Dorsey is tightening Twitter’s belt.

Just days after being named permanent chief executive of Twitter, Mr. Dorsey is planning a series of cost-cutting maneuvers at the social networking company, including layoffs and halting a plan to expand the company’s San Francisco’s headquarters, according to three people familiar with the plans, who spoke on the condition of anonymity because the details are private.

The plans are part of an effort to trim what many insiders see as an organization that has grown bloated — Twitter has over 4,100 employees in more than 35 offices — over the past few years, these people said.

A Twitter spokesman declined to comment.

The moves, with the layoffs set to come as soon as Tuesday, signal swift action by Mr. Dorsey, who was named permanent chief only on Monday. Mr. Dorsey, 38, who also co-founded Twitter, had held the interim chief title since July 1 and has had time to assess the company’s prospects, strengths and weaknesses.

He returned to a company in crisis. While Twitter went public in 2013 in a wave of hype, its stock price has more recently fallen — at one point dropping below its $26 initial public offering price — as the company has struggled to attract new users. Twitter faces intense competition for consumer attention from rivals like Instagram, Snapchat and WhatsApp, threatening its ability to attract new advertising dollars.