WASHINGTON — President Obama on Wednesday announced what he called perhaps his most important economic decision, nominating Janet L. Yellen to lead the Federal Reserve system and be his independent co-steward of the economy, calling her “one of the nation’s foremost economists and policy makers.”

Ms. Yellen, 67, would be elevated from the Fed’s vice chairwoman to become the first woman to lead the 100-year-old central bank. The Senate is generally expected to confirm her nomination for the four-year term.

For the announcement, she joined Mr. Obama in the State Dining Room of the White House, along with the retiring chairman, Ben S. Bernanke, whom the president hailed for helping guide the economy through the worst financial crisis since the Depression.

The president said Ms. Yellen was “renowned for her good judgment,” and he credited her with sounding early alarms about the financial and housing bubbles that caused the economy’s near-collapse in 2008.