The 27-year-old former Catalans Dragons back-row forward, who was left without a club following Hull KR's relegation from Betfred Super League, has signed a 12-month contract with the Tigers, who have an option to extend the deal by a further two years.
Larroyer, who had been training with Hull FC while searching for a new club, is being loaned to the Bulls to give him game time and Castleford will have the option to recall him at 24 hours notice after 28 days.
Tigers coach Daryl Powell said: "Kevin will add depth to our squad while we still have Larne Patrick and Oliver Holmes out injured.
"Kevin is a player who is proven at Super League level and did a really good job for Hull KR. He can play in the back row and in the middle unit as well which gives us some real depth and competition for places, which will be really valuable as we move into the Easter period."
Larroyer, who made the last of his 14 appearances for France against England in October, was in the Hull KR team that reached the Ladbrokes Challenge Cup final at Wembley in 2015.
"I'm absolutely delighted to sign for Castleford," Larroyer said. "To be part of this squad and this club is an honour.
"I've played at the Jungle a few times against Castleford and I've always thought the fans and the atmosphere were amazing. I'm looking forward to embracing this challenge and giving my best to Castleford Tigers."