BOSTON — Jerome H. Powell, the Federal Reserve chairman, said Tuesday that the American economy is enjoying an unusual but sustainable period of low unemployment and low inflation. He described the current moment, and the Fed’s expectation that it will continue, as “not too good to be true.”

Inflation is hovering around the 2 percent annual pace that the central bank regards as optimal while the unemployment rate has remained close to 4 percent for the last year. Economists have long regarded low unemployment as a harbinger of higher inflation, and there is no precedent in modern American history for both economic indicators to remain at such low levels.

But Mr. Powell said there was reason to believe this time could be different.

He said the Fed’s success in holding down inflation in recent decades has reinforced public expectations that inflation would stay low, and that, in turn, is helping to keep inflation low.

“These developments amount to a better world for households and businesses, which no longer experience or even fear the scourge of high and volatile inflation,” Mr. Powell said on Tuesday in Boston at the annual meeting of the National Association for Business Economics.