The Royals needed help at shortstop after Mike Aviles, who hit .325 as a rookie last year, went on the disabled list May 23 after batting .183 in 36 games. He underwent Tommy John surgery on his right elbow Wednesday and is expected to be out nine to 12 months.
Willie Bloomquist, who has played every position but pitcher and catcher this year, took over as the full-time shortstop.
"This is an upgrade for us," Royals manager Trey Hillman said of Betancourt. "I like it because it gives us a good shortstop with decent offensive stats."
The 27-year-old Betancourt is batting .250 with two homers and 22 RBIs in 63 games this season. He's been on a rehab assignment with Triple-A Tacoma after straining his left hamstring, and will continue rehabbing at Double-A Northwest Arkansas.
"We'd hope to have him back by the second half" of the season, Hillman said. "He makes us deeper and stronger overall."
The Mariners and Royals will be splitting the costs of the remainder of Betancourt's contract. He is owed $3 million next year; the Royals will pay $2 million of that. In 2011, he is owed $4 million, of which the Royals will take care of $3 million.
The Mariners will pay all of Betancourt's salary this season.
The Mariners receive right-hander Danny Cortes and left-hander Derrick Saito in the trade. Cortes will report to Double-A West Tenn and Saito will head to Class A Clinton.
Seattle manager Don Wakamatsu said Ronny Cedeno's play at shortstop allowed the club to make the trade.
"We have discussed it enough, starting in spring training, what our philosophy is and we're going to do whatever is possible to help us win here," he said. "We will continue to give guys an opportunity and we're giving Ronny Cedeno an opportunity right now."
Betancourt fled his native Cuba on a makeshift raft five years ago and signed with Seattle as a free agent in January 2005. He's hit .279 with 27 homers and 202 RBIs in 588 career games, and signed a three-year contract extension in April 2007.
He's struggled with plate discipline this season, drawing only 10 walks in 224 at-bats. He also was benched twice this season by Wakamatsu for lackluster play in the field.
"It's a new beginning," Betancourt said through an interpreter. "You have to keep your head up high and keep working."
The 22-year-old Cortes is 6-2 with a 3.12 ERA in his last 11 games, after starting the season 0-4 for Northwest Arkansas. He was rated by Baseball America as the Royals' top pitching prospect entering the spring.
The 21-year-old Saito is 2-6 with one save and a 4.26 ERA for Class A Burlington.
Information from ESPN The Magazine senior writer Buster Olney and The Associated Press was used in this report.