The 27-year-old former Arsenal player is manager Pierre Sage's second signing of the summer transfer window, following the arrival of defender Oscar Mingueza.
Tomiyasu, capable of playing at full-back and centre-back, spent the second half of last season at Dutch giants Ajax.
The defender, who has won 46 caps, was part of Japan's squad at the 2026 World Cup.
South London club Palace have enjoyed a golden spell over the past two seasons, winning the FA Cup, Community Shield and UEFA Conference League under former manager Oliver Glasner.
"I'm excited to get started," said Tomiyasu. "This club is in a good moment, winning three titles in two seasons. Here is where I wanted to be."
Palace 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."
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