The forward - who joined Albion from Almeria in January 2013 – leaves Albion for a club record transfer fee, believed to be £8million rising to £10million, to join the Premier League newcomers.
Ulloa hit 25 goals in 52 appearances for Albion, with last season disrupted by a foot injury during the first half of the campaign.
"He comes into quite a competitive squad. If it heightens the competitive spirit in the squad, then that's good," Leicester manager Nigel Pearson told Sky Sports.
"I think the players have a good collective spirit that has served us well so far, but we are going to have to replicate that this season, and also on the technical side of it make sure we continue to improve. I think we've got decent scope for that."
Albion chairman Tony Bloom revealed his club were very reluctant to sell the 27-year-old but reconsidered their position when the player himself let it be known he wanted to move.
"Initially we did not want Leo to go, but once he made it clear to us he wanted to join Leicester and play in the Premier League, it was then a case of ensuring we received our valuation," Bloom said.
"Leo has been tremendous for us, he has given us some truly memorable moments in the last season and a half - with his header against Nottingham Forest the pick of the bunch.
"I would like to thank him for his efforts and wish him well in the Premier League."
Leicester are scheduled to fly out to Thailand on Wednesday ahead of a friendly with Everton on Sunday in Bangkok.