It was the Spaniard's second win of the season, in five races, and the 32nd win of his career - a triumph that endorsed his challenge for a third drivers' title on an afternoon of Spanish supremacy at the Circuit de Catalunya.
Driving with great aplomb, Alonso delivered a beautifully-judged performance to win a tactical contest dominated by tyre-wear management, finishing 9.338 seconds ahead of second placed Finn Kimi Raikkonen of Lotus.
Alonso said: "What a fantastic feeling, you feel the support from everybody, we wanted to do well here in front of our fans, so many people supporting Ferrari."
Alonso's win bucked a trend at the Catalan circuit where 18 of the previous 22 events were won the driver who started from pole position. This time the pole-sitting German Nico Rosberg of Mercedes finished sixth ahead of Paul Di Resta of Force India.
Jenson Button finished a measured eighth for McLaren ahead of his team-mate Sergio Perez with Australian Daniel Ricciardo of Toro Rosso completing the top ten finishers.
Lewis Hamilton who started from second alongside Rosberg struggled hom 12th on a disappointing day for him and Mercedes.
In the drivers' championship, Alonso jumped to third with 72 points behind leader Vettel on 89 and Raikkonen on 85.
