The 21-year-old England U-21 international is new Crystal Palace manager Patrick Vieira’s fifth summer signing at the club.
Palace have already brought in Michael Olise, goalkeeper Remi Matthews, Marc Guehi and Joachim Andersen from Olympique Lyon.
Gallagher also spent last season on loan at relegated West Bromwich Albion, where he made 32 appearances and scored two goals.
He has also spent time on loan at Championship sides Charlton Athletic and Swansea City in the past.
Speaking after sealing his temporary switch to the Eagles, Gallagher said: "I'm buzzing to be here. I can't wait to get started, I'm happy to get the deal over the line.
"I spoke to the manager, I liked what he said, I liked how he wants to play, how he wants me to play and how he wants the team to play. I feel like I can learn from him and the players around me."
Palace chairman Steve Parish added: "I am delighted Conor has chosen to come to Crystal Palace on a season-long loan after some impressive performances in the Premier League last season."
"We very much look forward to him joining the squad as we continue to build towards the new season."
Gallagher will be unable to face his parent club when Crystal Palace visit Chelsea in their opening Premier League game on August 14.
