Marquez also won races at a new course in Austin, Texas, in April and last week in Germany, as he surpassed Freddie Spencer as the youngest to record consecutive victories.
Starting in second place on Sunday, Marquez negotiated a couple of nifty passes, including an overtake of Rossi on the treacherous Corkscrew corner in the early stages.
He then roared past Bradl to win the race by 2.2 seconds.
"I overtook Valentino then caught Bradl," he said. "I stayed behind him for about 12 laps, then tried to pass him to open up a gap."
Laguna Seca is the second of three MotoGP events on US soil this year as international motorcycle racing seeks to make a big splash and gain more North American fans.
Marquez now has a 16-point lead over teammate Dani Pedrosa in the overall points race at the season's halfway mark. Lorenzo is in third, 26 points adrift of Marquez.
LCR Honda's Bradl celebrated his a first podium finish from the pole position.
"I am so pleased with my result. The pressure was high this week," Bradl said. "My first podium here. We did good speeds all week."
The 34-year-old Rossi held off Alvaro Bautista for the final podium spot. Spain's Pedrosa rounded out the top five.
Rossi returned to Yamaha this year after a couple of frustrating seasons with Ducati.
Spain's Jorge Lorenzo finished sixth as he rode Sunday through the pain of a broken collarbone. He injured it in a crash at Assen, Netherlands in June and then re-fractured it in Germany.
Officials held a moment of silence before the start of the race to honour the memory of Italian Andrea Antonelli, who died after a crash during the World Supersport race in Moscow earlier on Sunday.
The second half of the MotoGP season begins in three weeks in Indianapolis.
-AAP
