Reigning title holder Josef Newgarden now sits second in the points behind Rossi, four-time Champ Car title victor Sebastien Bourdais is fourth, Scott Dixon, Will Power and Ryan Hunter-Reay lie sixth, seventh and eighth respectively, Tony Kanaan is 11th, and Simon Pagenaud is mired in 16th after being eliminated from Long Beach by an opening lap accident.
"We nailed the setup in each session this weekend", said Rossi.
After clinching the Verizon P1 award on Saturday for putting the vehicle on pole position by almost 0.5secs, Rossi was able to run an inspired two-stop strategy in the race which not one of his rivals was able to replicate.
American Alexander Rossi dominated the race from pole and took a comfortable win from Australian Will Power and Dixon's Chip Ganassi Racing team-mate Ed Jones. Rossi is the fifth driver to win at Long Beach from the pole, but the first since 2007.
Following a restart on Lap 76, Rossi kept Power at bay over the final 10 laps despite being at a disadvantage in push-to-pass, which allows the driver to activate a boost of turbocharger pressure and additional horsepower. Even when we saw him at Phoenix, on the oval [where Rossi finished third]. The 2016 Indianapolis 500 victor and veteran of five Formula One races hadn't won since Watkins Glen last year, but he had put his speedy Honda in position to win this year both in St. Petersburg - where he instead made late contact with rookie Robert Wickens - and again last weekend in Phoenix, where he finished third. "You can always look back and think of something you would do different, but this was pretty close to perfection".
Rossi completed the race in a time of 1:53.15 with an average speed of 88.622 miles per hour, beating runner up Will Power (#12 Team Verizon Penske) to the checkered flag by a 1.2 second margin.
"I think he's going to be tough to beat in the championship", Power said. "His auto was really, really good".
Rossi, in the No. 27 NAPA ATO PARTS Honda, led all but 14 of the 85 laps on the 1.986-mile, 11-turn course and crossed the finish line 1.2413 seconds ahead of Will Power. With the victory, Rossi climbed into the unofficial points lead after three of 17 races this season, holding a 22-point advantage over reigning series champion Josef Newgarden. Corrects Pagenaud's first name to Simon in third paragraph.
Dixon and Bourdais also got into further trouble after coming into the pit lane when it was closed after Zachery Clayman De Melo hit the wall on lap 60, forcing Kiwi driver to serve a drive-through penalty while Bourdais drove through without receiving service.
The race's next caution came out when Kyle Kaiser's No. 32 Juncos Racing Chevrolet stalled in the Turn 1 runoff area on Lap 42. Rahal got a drive-through penalty, but still rallied back for a strong finish.
However, the Frenchman was deemed to have entered the pit exit lane during the move and was ordered by the stewards to give the place back to Dixon.
Britain's Jordan King spoiled Bourdais' race in the latter stages after going into the back of the St.Petersberg victor at the final hairpin on lap 72 leading to a partial blockage of the track, this incident came after the rookie came into the pits to try and re-bleed the brakes after complaints of a long brake pedal.
Ryan Hunter-Reay also had to come in early for a new nose after opening-lap contact with Scott Dixon. Hunter-Reay fought his way back up to fifth, but got a puncture on contact with Jordan King.
The Verizon IndyCar Series returns to action Sunday, April 22 with the Honda Indy Grand Prix of Alabama at the scenic Barber Motorsports Park in Birmingham, Alabama.