Dempsey took a short pass from Jozy Altidore following Michael Bradley's cross and put a right-footed shot from just inside the penalty area just past an outstretched hand of goalkeeper Gianluigi Buffon. It was his 25th goal in 83 international appearances.
A native of Nacogdoches, Texas, who turns 29 on March 9, Dempsey has had the most accomplished season by a U.S. attacking player in European soccer. He's scored 16 goals this season for Fulham, becoming the American career scoring leader in England's Premier League with 43 goals.
The loss was particularly deflating for Italy, a four-time World Cup champion preparing for the start of the European Championship in June. The Americans, who are getting ready for their opening qualifier for the 2014 World Cup in June, had been 0-7-3 against Italy and had been outscored 32-4.
Italy dominated for stretches but the American defense held and the Azzurri were called for offsides nine times.
With retired baseball star Mike Piazza watching from the stands alongside former U.S. forward Brian McBride, the Americans won their fourth straight match to improve to 5-4-1 under Jurgen Klinsmann, who took over as U.S. coach from Bob Bradley last summer.
It was a measure of personal revenge for Klinsmann, the former German World Cup great. Germany, then coached by Klinsmann, lost to Italy in the 2006 World Cup semifinals and the Azzurri went on to win the title.
U.S. goalkeeper Tim Howard had a big kick save in the fifth minute. Italy nearly tied the score in injury time when Riccardo Montolivo's shot went past a post.
Italy lost in Genoa for the first time since 1924. The previous time the Azzurri played at Stadio Luigi Ferraris was a match stopped in the seventh minute by crowd trouble. Italy was awarded a 3-0 win over Serbia in a Euro qualifier.
Klinsmann had most of his regulars back following victories over Venezuela and Panama while playing with a backup squad. He had hoped to be able to pair Dempsey with Landon Donovan for the first time since taking over as coach, but Donovan missed the match because of bronchitis.
Italy had its own personnel problem, especially up front where it was missing injured forwards Giuseppe Rossi and Antonio Cassano. Manchester City striker Mario Balotelli was left of the squad because of his temper and unpredictability.
Notes: Dempsey became just the fourth American to score against Italy, joining Buff Donelli (1934), John Harkes (1992) and Landon Donovan (2009). There was also one own goal in 2006.