Perfectly placed after a narrow, twisting rise to the finish of the 155-km third stage in Luarca, the Quick Step rider outgunned race leader Oscar Freire in a close sprint, with Australian Allan Davis finishing third.
Freire, who won stage two of the three-week race, maintained his lead in the overall standings. Leonardo Duque of Colombia is second and Germany's Erik Zabel is in third with the same time as Freire.
''It's my first win since a stage win in the Tour of California in February and it couldn't have come at a better time.'' Bettini told reporters.
''The world championships (in Stuttgart, Germany on September 30) are my big objective for the end of the season, so this win is a welcome boost to my morale.'' For Bettini, the third Tour of Spain stage win of his career provided the perfect answer to Italian national coach Franco Ballerini's recent comments he had gone off the boil.
''Victories like this one showed what I can do, and in the best way possible -- on the bike,'' Bettini, 33, said.
Just 69 riders finished in the same time as Bettini after a short, sharp final climb in the fishing town of Luarca split the bunch.
Euskaltel-Euskadi leader Haimar Zubeldia fell as the bunch jostled for positions in the finale, badly bruising his left shoulder. Zubeldia, fifth in the Tour de France, finished 11 minutes down.
Spanish sprinter Freire is likely to lose the lead on the race's first major incursion into the mountains tomorrow, with the 14-km Lagos de Covadonga climb the biggest challenge.
''We won't see who can win the Tour of Spain, but we'll definitely see who's come here just to make up the numbers.'' Spanish favourite Carlos Sastre, fourth last year and second in 2005, predicted.
The Tour of Spain will finish on September 23 in Madrid.
REUTERS PY KP2330