ZURICH, Sept 22 (Reuters) - The Swiss National Bank raised its policy interest rate by 0.75 percentage point on Thursday - only the second increase in 15 years - as it joined other central banks in tightening monetary policy to curb rising prices.

The SNB increased its policy rate to 0.5% from the minus 0.25% level it set in June. Previously Swiss rates had been frozen at minus 0.75% for years as the SNB tried to tame the appreciation of the Swiss franc.

Most economists polled by Reuters had expected the SNB to raise its policy rate to 0.5%. (Reporting by John Revill; Editing by Michael Shields)