FRANKFURT—Germany's top representative on the European Central Bank resigned in an apparent protest Friday, dealing a severe blow to the steward of Europe's common currency amid the Continent's worsening debt crisis.

The ECB said Jürgen Stark, its chief economist, was resigning for "personal" reasons with nearly three years remaining in his term. But people familiar with the matter said his decision was driven by frustration over the bank's expanding role in backstopping the region's finances.

Mr....