The deal is worth $144 million, which would be the largest in Padres franchise history, according to multiple reports. The deal was first reported Saturday night by the San Diego-Union Tribune.
The contract is heavily front-loaded and includes an opt-out clause after the fifth year, according to reports. Hosmer will make $21 million annually over the first five years of the deal, according to FanRag Sports.
Hosmer, 28, joins a rebuilding Padres team that includes young sluggers Wil Myers and Hunter Renfroe. The Padres also have one of baseball's top-rated farm systems; San Diego's minor league system was ranked No. 3 overall last month by both ESPN's Keith Law and Baseball America.
With Hosmer, a four-time Gold Glove winner, at first base, the Padres likely will move the 27-year-old Myers back to the outfield. Myers had been San Diego's primary first baseman over the past two seasons after starting his career as an outfielder.
Hosmer batted a career-high .318 last season with the Kansas City Royals and matched his best from the previous season with 25 homers. He also had 94 RBIs, scored 94 runs and finished with a career-best .385 on-base percentage.
Hosmer was among the top free agents in the offseason who remained unsigned into the start of spring training. Right-hander Yu Darvish signed with the Chicago Cubs earlier this month, but Jake Arrieta and outfielder J.D. Martinez are still without contracts.
Hosmer is the latest addition by the Padres this offseason. Previously, they reacquired third baseman Chase Headley from the New York Yankees and got shortstop Freddy Galvis from the Philadelphia Phillies.
Hosmer is one of four core Royals players, along with Mike Moustakas, Lorenzo Cain and Alcides Escobar, who hit free agency for the first time this year. They arrived in Kansas City in 2011 as a group of 20-somethings and keyed the team's run to consecutive World Series, including a championship in 2015.