His No.1 Renault crossed the finish line 14.5 seconds ahead of McLaren's Juan Montoya of Colombia.
It was Alonso's 4th victory of the season and 12th of his Formula One auto racing career.
"First of all I'd like to dedicate this victory to Eduoard Michelin," Alonso said, referring to the tire manufacturer who drowned in a boating accident on Friday. "Monaco was one of the races that was a cross on my calendar. It feels great to win again so soon after Barcelona (and) Michelin gave us the tools to beat our competition this afternoon."
Alonso was given the pole position when it was determined that Michael Schumacher deliberately stopped on the circuit during the qualifying session.
The front-running Alonso increased his lead over Schumacher in the points race to 43.