The Japan international has joined Palace on a free transfer, becoming manager Pierre Sage's second signing of the summer after Oscar Mingueza.
Tomiyasu, 27, can play at both full-back and centre-back and spent the second half of last season with Dutch giants Ajax. He has won 46 caps for Japan and was part of their squad at the 2026 FIFA World Cup.
The defender arrives at Selhurst Park as Palace look to build on a hugely successful two seasons. The club have won the FA Cup, Community Shield and UEFA Conference League during that period.
Tomiyasu played for Crystal Palace in July 2026 before his official announcement because he was with the club on a pre-season trial period as a free agent.
Tomiyasu said he was excited about joining a club that has enjoyed such a strong run of success.
"I'm excited to get started," he said. "This club is in a good moment, winning three titles in two seasons. Here is where I wanted to be."
Palace chairman Steve Parish welcomed the defender's experience at the highest level.
"Takehiro has experience at the highest level of European and international football," Parish said. "We are delighted to welcome a player of his calibre to the club."
Crystal Palace play their next match against SC Freiburg in a pre-season club friendly on Saturday, 15 August at Europa-Park Stadion in Freiburg.
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