"Really, words can't describe it right now," Lester said.
Lester (3-2) allowed just two baserunners, walking Billy Butler in the second inning and Esteban German to open the ninth.
Lester struck out nine, fanning Alberto Callaspo to end the game before pumping both fists in the air. It was Lester's first major league complete game, and what a way to do it.
Catcher Jason Varitek, who has been behind the plate for a record four no-hitters, lifted his pitcher into the air.
Then Lester, pitcher, who missed the end of the 2006 season after he was diagnosed with a rare form of non-Hodgkin's lymphoma, met manager Terry Francona met for a long, hard embrace.
"He just said he was proud of me," Lester said. "I've been through a lot the last couple of years. He's been like a second dad to me. It was just a special moment right there."
Center fielder Jacoby Ellsbury made a diving catch of Jose Guillen's line drive to end the fourth - the best defensive play of the night. Lester also got help from first baseman Kevin Youkilis, who made a nice scoop on shortstop Julio Lugo's throw after David DeJesus hit a grounder in the third.
The fans at Fenway Park really got into it for the final out of the seventh, rising to their feet when Lester fanned Guillen on a 93 mph fastball. They remained standing for the entire ninth inning, even as German walked and moved around to third base when Tony Pena and DeJesus grounded out.
With flashes popping and the fans screaming at full throat, Callaspo fell behind 0-2. He fouled one off and took a ball before reaching for a high and outside 1-2 fastball to end the game.
Lester and Varitek were mobbed by teammates running out of the dugout as the speakers played "Tessie," the victory anthem the Red Sox adopted through two World Series titles in four seasons. Lester was instrumental in the second, earning the victory in Game 4 at Colorado less than a year after chemotherapy cured his cancer.
After Lester hugged Francona, the pitcher tipped his hat to the fans for one more big cheer.
"He's not just a good kid because he threw a no-hitter," Francona said. "He's a good kid because he's a good kid."
Boston's last no-hitter was pitched by Clay Buchholz, who shut down the Baltimore Orioles in just his second major league start last Sept. 1.
Mel Parnell was the last Red Sox lefty to throw a no-hitter, beating the Chicago White Sox on July 14, 1956. Hall of Famer Nolan Ryan is the only other person in history to no-hit the Royals.
"Just trying to finish the game," Lester said in an on-field interview. "I can't lie, it went through my head a couple times in the ninth."
Luke Hochevar (3-3) allowed seven runs on five hits and six walks in six innings for Kansas City, which had won six of its last seven.
The Red Sox took advantage of Hochevar with five runs in the third, scoring their first run on a double play and then loading the bases on walks after Ellsbury tripled. Manny Ramirez walked to bring in another run and then Mike Lowell hit a high popup that was dropped by second baseman Mike Grudzielanek for another two runs.
Youkilis bounced a line drive over the short wall in right to make it 5-0. Varitek added a two-run homer in the sixth.
