The Securities and Exchange Commission on Friday removed the chairman of the U.S. audit watchdog and took steps to replace its entire board, as the SEC's new Democratic leader begins to shape the regulator.

The SEC, which oversees the Public Company Accounting Oversight Board, said the departure of William Duhnke was effective immediately and that it appointed board member Duane DesParte to serve as acting chair. It didn't provide a reason for the changes. Mr. Duhnke couldn't be reached for comment.

The PCAOB regulates the audits of publicly traded companies. Congress established it with the Sarbanes-Oxley Act of 2002 following accounting scandals at Enron Corp. and other now-defunct firms. Mr. Duhnke was sworn in as PCAOB chairman in January 2018 under then-President Donald Trump.

The SEC's new Democratic chairman, Gary Gensler, took control in April. A former banker and regulator, he is expected to be tougher on companies and look to increase investor protections and reporting requirements, for example around disclosures about environmental, social and governance issues.

"The PCAOB has an opportunity to live up to Congress's vision in the Sarbanes-Oxley Act," Mr. Gensler said in a statement. "I look forward to working with my fellow commissioners, Acting Chair DesParte, and the staff of the PCAOB to set it on a path to better protect investors by ensuring that public company audits are informative, accurate, and independent."

Mr. Duhnke in recent months has faced criticism from investors who said the PCAOB didn't adequately incorporate their feedback into its work. Under Mr. Duhnke, the watchdog in 2018 stopped holding meetings of two groups that consulted investors, citing what the board described as the meetings' ineffectiveness. It still conducts other events with investors though.

U.S. Senators Elizabeth Warren (D., Mass.) and Bernie Sanders (I., Vt.) last month wrote a letter urging the SEC to remove all members of the PCAOB. They said the Trump administration took deliberate steps to erode the PCAOB's independence and expertise, leading the watchdog to, for example, weaken auditor-independence rules and exclude investors from participating in its policy-making process.

One consequence of the SEC's decisions on Friday could be the overhaul of the entire PCAOB board. The regulator said it would seek candidates to fill all five board positions on the PCAOB in the coming weeks. One seat is vacant after J. Robert Brown left the PCAOB in January. The remaining three members would remain in their positions pending the outcome of the SEC's search and can apply to stay on the board.

The PCAOB under Mr. Duhnke sought to overhaul its largely outdated quality-control rules and streamline how it conducts inspections of audit firms. The board since 2019 has considered revisions to its standard on audit firms' internal controls related to audit quality, but it has yet to issue a proposal.

Before joining the PCAOB, Mr. Duhnke worked as the majority staff director and general counsel to the U.S. Senate Committee on Rules and Administration, then led by Sen. Richard Shelby (R., Ala.).

Mr. DesParte has served as a PCAOB board member since April 2018. Before the PCAOB, he was corporate controller at energy company Exelon Corp.

The SEC's decisions on Friday were a necessary step to ensure the PCAOB maintained credible audit reports, said Lynn Turner, a former SEC chief accountant.

Meanwhile, SEC commissioners Elad Roisman and Hester Peirce, both Republicans, criticized the decisions, stating they have "serious concerns about the hasty and truncated decision-making process underlying this action." Michael Piwowar, a former Republican member of the SEC from 2013 to 2018, said the removal of Mr. Duhnke politicizes the PCAOB. "This is terrible and unprecedented," he said.

The SEC replaced the board in 2017, after a scandal involving the leak of confidential inspections data.

A PCAOB spokeswoman declined to comment beyond the SEC's announcement.

Write to Mark Maurer at mark.maurer@wsj.com