"I went up there looking for a knuckleball and it stayed up high and I put a good swing on it," Betancourt said through a translator. "I knew I had at least a double but I wasn't sure it would get over the wall."
David DeJesus had four of the 20 hits for the Royals, leading the American League in hitting (.278) entering Friday, off six Boston pitchers, including utilityman Bill Hall, who made his first big league appearance as a pitcher.
Kansas City had five straight singles to open the fourth, and with two out Betancourt lofted a knuckleball from Wakefield into the seats above the Green Monster in left for the second grand slam of his career as the Royals beat Boston for the second straight night.
The Royals pummeled Wakefield (1-3) for 12 hits and nine runs in just 3 2/3 innings.
"When I left the bullpen I thought I had really good stuff," Wakefield said. "I had good stuff in the first inning. I just wasn't able to repeat that."
Mitch Maier had three hits for the Royals, including his first home run of the season.
Royals Manager Ned Yost said he thinks it's about time his team got some respect for being a good offensive club.
"Sooner or later, we will," he said. "To me, since I've been here, this club has put together good at-bat after good at-bat."
Royals starter Kyle Davies (4-3) worked five shaky innings for his fourth win.
"He found a way to get through five innings to his credit," Yost said.
The Red Sox scored three times in the bottom of the first. Marco Scutaro doubled down the left field line and, with one out, David Ortiz took advantage of the shift and rolled an RBI single into left. After Kevin Youkilis walked, Victor Martinez, returning to the lineup for the first time since hurting his toe Monday, hit a two-run double to the 420-foot mark in right-center.
The 43-year-old Wakefield, who returned to the rotation when Josh Beckett went down with a back injury on May 17 and beat the Phillies in his last start, wriggled out of a bases-loaded jam in the second by getting Scott Podsednik to fly out, but the Royals struck for two runs in the third.
Mike Aviles singled and rolled home on a double to left-center by David DeJesus. With one out, Jose Guillen walked and Sox third baseman Adrian Beltre, after getting one out, threw wide to first on what might have been an inning-ending double-play grounder by Alberto Callaspo. Maier followed with an RBI single.
After Youkilis walked again in the third, Martinez lined his seventh home run into the Royals' bullpen in right field to give the Red Sox a 5-2 lead.
The Royals are 9-5 since Yost took over as manager. ... The Red Sox placed outfielder Jacoby Ellsbury on the 15-day disabled list Friday after he continued to complain of soreness and pain in the rib cage area. ... Red Sox manager Terry Francona said there was no timetable for Ellsbury's return. "We hope it's quick, but we don't know." The Red Sox recalled pitcher Scott Atchison from Pawtucket to take Ellsbury's place on the roster. ... The Royals' DeJesus extended his hitting streak to 10 games. ... Youkilis walked for the 30th and 31st time in May and reached base for the 27th straight game.