The dollar kicked off 2015 by rallying against other major currencies, underscoring heightened expectations for robust U.S. growth in the new year relative to the rest of the world.

The WSJ Dollar Index, which tracks the greenback’s performance against 16 other currencies, on Friday jumped to an 11 1/2 -year high.

Investors are piling into the dollar in anticipation of the Federal Reserve’s first rate increase in almost a decade, a move that many investors, strategists and economists expect to come sometime this year. The U.S. central bank is seeking to return to more standard monetary policies as the country’s labor market and economy have improved steadily since the financial crisis. Higher benchmark interest rates tend to boost a currency as investors, in search of bigger returns, convert cash into that currency.

At the same time, central banks in other major economies, such as Europe and Japan, are signaling that they are likely to expand easy-money policies amid flagging growth and concerns that deflation, a potentially damaging cycle of price and wage decreases, could take hold.

“U.S. economic outperformance is at the forefront of this rally,” said Alan Ruskin, head of G-10 foreign exchange strategy at Deutsche Bank in New York. “This year will likely be when we finally see the rate hikes that will separate the U.S. from much of the world.”

This divergence is drawing money managers from around the world to dollar-denominated assets and pushing up the dollar’s value. The stronger currency is having ripple effects in other financial markets. It has bolstered the allure of U.S. stocks and bonds, by boosting returns for investors who measure their performance in other currencies. A stronger greenback has also contributed to lower oil and gasoline prices, which have enhanced the spending power of U.S. consumers. Consumer spending drives U.S. growth.

The rallying dollar weighed on many raw-materials markets on Friday. Dollar-denominated commodities become more expensive for buyers using other currencies when the buck appreciates, which saps global demand for the materials. The Bloomberg Commodity Index, which fell for the fourth-straight year in 2014, recently was 0.5% lower.

The dollar on Friday rose 0.8% to a 4 1/2 -year high against the euro. Underlining the contrasting stances of the Fed and the European Central Bank, ECB President Mario Draghi told German newspaper Handelsblatt that the central bank is preparing further easing measures if they prove to be necessary.

Preparations are being made to “alter the volume, tempo and content of measures early in 2015, if needed, to respond to a period of inflation that is too low,” Mr. Draghi said in an interview published on Friday. Mr. Draghi’s words fanned speculation that a stimulus program involving the purchase of sovereign debt could be implemented sooner rather than later. Recently, one euro bought $ 1.2008, compared with $ 1.2099 late Wednesday in New York.

The dollar notched a 0.5% gain against the yen, hovering near seven-year highs. The dollar rose 1.5% against the U.K. pound to the highest level since August 2013, with sterling recently buying $ 1.53. The greenback also traded at parity with the Swiss franc for the first time in more than four years on Friday.

Speculators have increased their pro-dollar bets by 37% since the beginning of 2014, according to Société Générale.

“One way or another, investors are taking every opportunity they can to get into the dollar trade,” said Sebastian Galy, a strategist at Société Générale. “It is drawing funds from central banks, hedge funds, even retail investors.”

To be sure, the dollar rally could stumble. Some investors question how long the U.S. economy can thrive while Europe, Japan and China languish. Manufacturing activity in China, the world’s second-largest economy, expanded at its slowest pace in 1 1/2 years in December, the country’s official manufacturing purchasing managers index showed Wednesday. China’s economy grew at its slowest rate in more than five years in the third quarter, and the full year could fall below the government target of about 7.5%.

Activity in the eurozone’s manufacturing sector grew at a slower pace than first estimated in December, the latest data from Markit showed Friday. U.S. manufacturers tapped the brakes in December as well, with data from the Institute of Supply Management showing the purchasing managers index falling in the month.

Investors also risk misreading signals from the Fed, which may be less eager to raise rates than some expect, said Omer Esiner, chief market strategist at Commonwealth Foreign Exchange, a foreign-exchange broker based in Washington, D.C.

“The Fed has consistently played up downside risks to the economy, while markets tend to focus on the positives,” Mr. Esiner said. “There is potential for the Fed to disappoint the markets in terms of follow-through.”

Still, most investors expect the main driver of the dollar’s rally–diverging rates between the U.S. and other economies–to remain in place. Investors expect U.S. lending rates to top those in Germany and other major economies for at least the next three years, an indication that the dollar will likely continue soaking up the world’s cash, said Mr. Ruskin, of Deutsche Bank.

“It’s won’t be prudent to bet against the dollar too early,” he said.

Write to Ira Iosebashvili at

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