WASHINGTON — Stanley Fischer, vice chairman of the Federal Reserve, will step down in mid-October after three years at the central bank, the Fed announced on Wednesday.

Mr. Fischer, 73, cited “personal reasons” in a short resignation letter to President Trump.

The departure would leave just three people on the Fed’s seven-seat board, increasing the pressure on the Trump administration and Congress to fill some of the vacancies.

Mr. Trump has nominated Randal K. Quarles as the Fed’s vice chairman for supervision — a different job than that held by Mr. Fischer — but the Senate has yet to vote.

Mr. Fischer is leaving eight months before the end of his four-year term. He joined the Fed’s board in May 2014, shortly after Janet L. Yellen was promoted from vice chairwoman to chairwoman. Mr. Fischer brought a background as a distinguished academic economist and a widely respected policy maker.

He was a professor at the Massachusetts Institute of Technology for two decades, where his students included Mario Draghi, now president of the European Central Bank, and Ben S. Bernanke, Ms. Yellen’s predecessor as Fed chairman.

Then, in the late 1980s, he embarked on a second career in public service, first as chief economist at the World Bank, then as a senior official at the International Monetary Fund and as governor of the Bank of Israel.

At the Fed, Mr. Fischer has voted consistently with Ms. Yellen, but in his public remarks he has sometimes raised questions about whether the Fed is moving too slowly to wind down its post-crisis stimulus campaign.

“Stan’s keen insights, grounded in a lifetime of exemplary scholarship and public service, contributed invaluably to our monetary policy deliberations,” Ms. Yellen said in a statement. “I’m personally grateful for his friendship and his service. We will miss his wise counsel, good humor and dry wit.”

If Mr. Quarles is confirmed, Mr. Trump would still have three more Fed vacancies to fill. Administration officials earlier this year settled on Marvin Goodfriend, an economist at Carnegie Mellon University, as their choice for one of the Fed vacancies. But Mr. Trump has yet to nominate Mr. Goodfriend.

Mr. Trump also must decide whether to replace Ms. Yellen, whose term ends in February. He has said that his chief economic adviser, Gary D. Cohn, is also a candidate.