Needing pitching depth, the Royals called up Smith -- rated their No. 15 prospect by MLB.com entering the season -- from Triple-A Omaha on Tuesday.
At the same time, the club placed second baseman Chris Getz on the 15-day disabled list with a left ribcage bruise, an injury he aggravated during Monday night's 6-0 victory over the Yankees. The move gives the Royals a 13-man pitching staff.
Smith had a 1-3 record and a 4.01 ERA in nine starts this year for the Omaha Storm Chasers. He got the nod over two other promising lefties, Mike Montgomery and Ryan Verdugo.
"We want to see what Will Smith can do," Royals manager Ned Yost said. "His numbers aren't, like, eye-popping, but he's been throwing better. He developed a slider last year, and he's starting to put it all together. He's got a lot of maturity and composure."
Last season, Smith was 13-9 with a 3.85 ERA in 27 starts for Double-A Northwest Arkansas and was named the Naturals' Pitcher of the Year. The development of a slider was a major step for him.
"That gave me another pitch for left-handed batters," Smith said. "Now I've got a curveball and a slider, and that's really opened things up for me."
The 6-foot-5, 22-year-old Smith was obtained from the Angels with pitcher Sean O'Sullivan on July 22, 2010, in exchange for third baseman Alberto Callaspo.
To make room on the 40-man roster for Smith, the Royals placed pitcher Blake Wood on the 60-day disabled list. Wood is scheduled to undergo Tommy John surgery on his right elbow.
Smith got the good news after Omaha's 4-2 loss to Salt Lake on Monday night at home.
"We played our game and were all out eating the spread, and [manager Mike Jirschele] called me in, asked me how I felt," Smith said. "I said I felt fine and he said, 'You're going up.' And that was it.
"He shook my hand, gave me a hug and [pitching coach Doug Henry] did the same. And I was out of there."