WASHINGTON—Private economists kept their outlook for U.S. interest rates largely unchanged over the past month, with most expecting the Federal Reserve to continue tightening policy through next year.

All 58 economists who participated in The Wall Street Journal’s latest survey saw the Fed raising interest rates by a quarter of a percentage point, to a range between 2.25% and 2.5%, in December.

Their median forecast calls for three additional increases in 2019 to a range between 3% and 3.25%, where rates are projected to remain through 2021.

The economists’ outlook differs slightly from the median projection of Fed policy makers, who in September expected to raise their benchmark federal-funds rate to between 3.25% and 3.5% in 2020 and leave it there through 2021.

The Fed is gradually raising rates to keep the rapidly growing economy from fueling excessive inflation or dangerous asset bubbles.