By Neil MacLucas and Andrew Morse 

ZURICH--Switzerland's central bank on Thursday said it would introduce negative interest rates next year, a measure designed to cool the strength of the Swiss franc and ward off deflation.

Beginning Jan. 22, the Swiss National Bank will charge banks 0.25% to deposit overnight funds with it, the central bank said in a statement. The move will push the three-month Swiss franc Libor rate, currently in a range between 0.0% and 0.25%, into negative territory.

The SNB's decision comes after months of pressure on the franc, which has strengthened to near 1.20 a euro, a level the central bank has pledged for the past three years to defend. A strong franc, which has benefited from safe-haven buying and weakness in the eurozone economies, raises the risk of imported deflation and creates headwinds for the country's exporters, many of whom depend on the European Union as a key market.

The franc's appreciation against the euro in recent days had forced the central bank to intervene in the currency markets, SNB President Thomas Jordan said at a news conference, although he declined to say how much currency the bank bought. The SNB "can push the negative rate even lower" if needed he said and reiterated the bank's commitment to defend the 1.20-franc-to-euro level with what whatever measures needed.

The Swiss franc immediately dropped on the news, falling to 1.2098 a euro, its lowest level since October. The decision, made before the stock market opened, also buoyed Swiss shares with the benchmark SMI trading 1.8% higher.

The SNB's move comes as central banks around the world wrestle with diverging economies and problems. Europe remains caught in an easing phase as it battles the specter of deflation, while the U.S. is starting to consider raising rates for the first time in six years.

In June, the European Central Bank introduced negative interest rates on deposits with it, a move designed to encourage banks to lend rather than park money. The central bank of Denmark, which isn't part of the eurozone, has also used negative interest rates.

Overnight, however, the U.S. Federal Reserve signaled it would raise short-term interest rates, which have been at near zero since December 2008, sometime next year.

Like other central banks, the SNB is fighting a low-inflation environment that may be exacerbated by falling oil prices. Last week, the SNB said it expects the country to fall into deflation next year, a state that can slow economies by discouraging spending. Mr. Jordan said the SNB is "trying to avoid negative inflation extending over a long period" by introducing negative rates.

The SNB is forecasting mild deflation of an average of -0.1% over the course of 2015.

"The SNB has bowed to the inevitable," said Kit Juckes, macro strategist at Société Générale. "If 2015 brings more ECB easing and the start of Fed tightening, it is going to be difficult to hold it and this may not be the last step they take."

The SNB's move will widen the range for three-month Swiss franc Libor, a key interest rate, to minus 0.75% to 0.25%.

Negative interest rates are unusual though not unprecedented. In 1972, the SNB introduced negative interest rates on nonresident bank balances at Swiss banks, part of an effort to stem a flow of foreign money into Switzerland. Mr. Jordan said Thursday's decision wasn't comparable because it was aimed at deterring inflows.

The move is the first change to the SNB's three-year policy of defending its minimum exchange rate through the purchase of euros, a practice which has seen its foreign currency reserves swell to more than 460 billion Swiss francs ($473 billion).

The SNB has insisted in recent months that it wouldn't exclude the use of negative rates to discourage investor buying of the franc, but some analysts expected the central bank to wait for the next move from the ECB.

The Swiss franc's "price action over the last few days suggest the SNB might have had to purchase material amounts of euros to defend the 1.20 floor and this may have triggered the move today," said UBS currency strategist Beat Siegenthaler.

The SNB said the 0.25% fee will be charged on Swiss franc sight deposit balances that exceed a certain threshold, which will vary with account holders, but will be at least 10 million francs.

Chiara Albanese in London contributed to this article.

Write to Neil MacLucas at neil.maclucas@wsj.com and Andrew Morse at andrew.morse@wsj.com

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