According to The Associated Press, Jay's deal is worth $3 million and he can earn an additional $1.5 million if he meets performance incentives. He would get $100,000 bonuses for 250 plate appearances and each additional 25 through 600.
Still, his earnings will be well under his $8 million salary last year.
To make room for Jay on the roster, the Royals placed pitcher Jesse Hahn on the 60-day disabled list.
Jay became a quiet leader for the Chicago Cubs in 2017 while slowly earning more playing time, eventually becoming the de facto leadoff hitter. Jay hit .296 with a solid .374 on-base percentage, though with little power. He's a solid addition to a team but not a front-line player.
Behind the scenes, Cubs players followed Jay's lead, as he rarely enjoyed talking about himself. He was a good mentor to up-and-coming center fielder Albert Almora Jr.
Jay can play all three outfield positions, though he isn't considered a Gold Glove-type defender. But like his offense, his defense is steady. He made several running catches in center field, proving that he can still play there and not just the corners. He's a good fourth outfielder but was more than that in 2016.
ESPN's Jesse Rogers contributed to this report.