The 32-year-old joins Palace on a two-and-a-half year deal from Women's Super League club Everton.
Ladd brings a wealth of experience to Palace, having previously represented Arsenal, Finnish side Kokkola Futis 10, Coventry City, Bristol City, Birmingham City, Manchester United and most recently, Everton.
She lifted the Adobe Women's FA Cup in 2023 with Manchester United, where she also won the club’s Player of the Season award in 2019/20.
Ladd is the Wales vice-captain, and currently stands as the fourth-most capped player in their history with 111 senior appearances.
She featured prominently during Wales' historic UEFA Women's EURO 2025 campaign last year, alongside Palace's Elise Hughes and Josie Green.
Ladd will be eligible to face Sheffield United on Monday evening.
Manager Jo Potter said: "I'm delighted Hayley is joining us - she is a player of real quality.
"We’re signing a top midfielder and a player whose leadership and experience will benefit the whole squad."
Reflecting on her move to Palace, Ladd said: "Yeah, it feels amazing. I'm so glad to be here.
"It's obviously a club that I've admired for a long time, so I'm thrilled to be here and ready to get started."