El Ghazi was initially suspended by the Bundesliga club last October over a social media post about the conflict and Mainz then terminated the player's contract in November. A German court ruled last month that his contract was wrongfully terminated.
"'So, surely with hardship comes ease' after the darkness of the last few months, I look forward to this new chapter at Cardiff," El Ghazi posted on social media platform X.
"I will forever remember your faith and love. Whatever the outcome, I will give my all for you on the pitch and do my best to deliver success and happiness. May the coming months bring prosperity and peace for all, including those in Gaza."
El Ghazi who only made three appearances for the German club, has previous experience in the Championship with Aston Villa, helping the club win promotion to the Premier League in 2019.
