MILAN—Wall Street's charging-bull statue is the very epitome of financial strength. Outside the stock exchange in Italy's financial capital is an equally unambiguous symbol: a giant marble middle finger.

The 36-foot white sculpture is a hand without fingers except that vulgar one, pointing away from Milan's stock market. It was made by Italy's most famous contemporary artist, Maurizio Cattelan, who donated it to the city last year on the condition that it grace Piazza Affari,...