TOKYO—Japanese banks are becoming an increasingly important source of finance for companies in the U.S. and Europe, as many domestic lenders are squeezed by write-offs from recent financial crises and a crunch on capital.

The trend represents a reversal of fortune for Japan's banks, which for years have struggled to keep up with their international rivals amid sluggish growth at home and the aftereffects of their struggles with bad loans in the 1990s.

...