The 27-year-old defender joins after a successful trial period and becomes Manager Pierre Sage’s second signing of the summer, following Óscar Mingueza.
Capable of playing at full-back or centre-back, Tomiyasu has previous experience in the Premier League, having spent four years with Arsenal.
The defender spent the second half of last season with Dutch side Ajax and was part of the Japan squad at the 2026 FIFA World Cup this summer.
At international level, Tomiyasu has 46 caps for his country to date. He will wear the No. 17 shirt for the Eagles.
Chairman Steve Parish said: “Takehiro has experience at the highest level of European and international football.
“We are delighted to welcome a player of his calibre to the club and believe he will prove to be a great addition to the squad.”
Tomiyasu said: “I’m excited to get started. This club is in a good moment, winning three titles in two seasons. Here is where I wanted to be.”