The lock forward has joined on a season-long loan from the Ospreys and follows Dmitri Arhip and Jason Harries in arriving at Wales’ capital region.
Thornton, 23, saw his game time limited last season as injury restricted him to just four appearances at the Liberty Stadium.
Following discussions between both regions and Wales’ national team management, it was agreed the former Wales Under-20 captain will make the temporary move up the M4.
The aim is for the transfer to allow Thornton the best opportunity to get more game time under his belt and to assist his international ambitions going into a Rugby World Cup year.
“I need to be playing rugby and Cardiff need a line-out forward so this is a great outcome for everyone,” Thornton said.
“It’s a good move for me in the circumstances and I’m looking forward to meeting the rest of the squad next week and preparing for an important season.
“I’m grateful to the Ospreys for supporting me like this and also to Cardiff Blues for giving me the opportunity.”
Thornton has one senior Wales cap to his name, making his debut against Samoa in Apia in June 2017.
Cardiff Blues head coach John Mulvihill welcomed the player’s arrival.
“It’s great to welcome a player of Rory’s calibre to Cardiff Blues,” he said.
“I have met him a few times now and I’ve been impressed with his desire to play international rugby again. He will get his opportunity to do this playing at Cardiff Blues.
“The support and co-operation by the WRU, Cardiff Blues and the Ospreys shows Welsh rugby is heading down a path where their best players will get the opportunity to play every week at the highest level.
“This collective mindset will ensure strength and depth at a national level going forward.
“From a Cardiff Blues perspective, it is important we have strength in depth in the lock position and Rory is exactly the right type of lock to add to what we already have.”
Allen Clarke, Ospreys head coach, added: “The decision to allow Rory to go to Cardiff Blues on loan has been made in his best interests and with his international ambitions in mind.
“A talented young player like Rory will benefit from playing regular rugby. He has ambitions of going to the World Cup and second row is a position where we have a number of options available. I want him to go to Cardiff and shine, knowing we will welcome him back next year.
“It’s important for Welsh rugby to work collectively to ensure the best outcome for individual players, regions and national team alike.”
