The 21-year-old, who has featured for Nigeria at U21 level, scored for Cardiff's development side against Coventry earlier this month, prior to the completion of his move.
Ajayi is yet to make a first-team appearance for the Gunners, although he has been an unused substitute in three Premier League matches this season.
He played three games on loan with Conference side Dartford in the 2012-13 season, scoring once.
Russell Slade said of the defender: “He's only 20 and very much a work in progress for us.
"He won't be near the first team this season. He will train with the senior squad and under-21s.
"He's an interesting player and exciting for his age. It's definitely something we will look at in summer. But let's see how things go."
Delighted to have joined @CardiffCityFC on loan, looking forward to what the rest of the season has in store.
— Semi Ajayi (@SemiAjayi) March 25, 2015