While Ajayi has been a regular for the Arsenal under-21s this season, he has yet to make his debut in the senior squad.
However, he was an unused substitute in Arsene Wenger's matchday squad for three Premier League fixtures this term.
The Nigeria under-21 international joined the Gunners in 2013 after coming through the academy at Charlton Athletic.
His contract at Arsenal expires in June and this loan move could pave a way to a permanent deal at Cardiff at the end of the season.
Good luck to Semi Ajayi who joins on loan until the end of the season! Story:
— Arsenal FC (@Arsenal)