TOKYO—Japan’s central bank and its main government pension fund said Friday they would pump trillions more yen into the country’s sputtering economy, taking a risky new stimulus tack that jolted global markets.

Faced with fresh evidence that Prime Minister Shinzo Abe’s campaign to end the country’s long bout with deflation was faltering, the two institutions steered Japanese economic policy into the uncharted territory of extreme stimulus that appeared to go even beyond the often radical measures taken by other advanced economies...