The 26-year-old has signed a four-year contract, subject to a work permit.
Medel completed his medical on Saturday after having featured for Sevilla in their 3-1 win at Manchester United on Friday.
Cardiff City manager Malky Mackay told the club's official website: "We're delighted to have secured the services of Gary - a tenacious and intelligent international midfielder.
"Having had two seasons of regular football in La Liga - with 60 odd international caps to his name and three appearances at a World Cup - his experience will be an invaluable asset to our squad.
"He's played in a number of Boca v River Plate derbies in Argentina - one of the world's biggest derby matches - and I've no doubt that his background and pedigree will complement our midfield group in the Barclays Premier League."