The 35-year-old midfielder joins Palace on a one-year deal, following her departure from Shamrock Rovers in the summer.
Littlejohn brings a wealth of experience to South London, having represented teams like Glasgow City, Arsenal, Liverpool, Celtic, Aston Villa and London City Lionesses throughout her 18-year career.
She made her Ireland debut in 2012 against Hungary and has since earned 91 caps for the Girls in Green. She played in Ireland’s most recent friendly against the USA in June, alongside her new Palace teammates, Abbie Larkin and Hayley Nolan.
Littlejohn managed to win plenty of silverware throughout her multiple spells at Glasgow City, winning the Scottish FA Cup in 2009, 2012 and 2013 and the Premier League Cup in 2009 and 2013.
Littlejohn will be eligible to face Nottingham Forest on Sunday.
Manager Jo Potter said: “We're delighted to bring Ruesha to the club. Her experience and winning mentality are exactly what we need, and she's a fantastic professional.”
Speaking on her move to Palace, Littlejohn said: “It feels great to be here, I’m buzzing to just get started!”