The former Wales Under-18s, 20s and Sevens international was signed by Richard Cockerill last summer on a one-year contract, having scored 22 tries in 45 appearances for London Scottish.
However, the 29-year-old has now opted to return to his homeland and is relishing the next chapter of his career at Cardiff Arms Park.
Harries said: “I’m really looking forward to linking up with the squad and getting stuck in to what will be a tough pre-season.
“This is the perfect opportunity for me as I take the next step in my career. Having qualified for the European Champions Cup and won the Challenge Cup it is an exciting time to join up with the Blues and the squad must be brimming with confidence.
“There is a huge amount of competition within the squad but that is only healthy and will lead to us being better both as individuals and a team, which is the ultimate goal.
“I am excited to show what I am all about and contribute to the region as we look to build on the success of last season.”
Standing at 6ft 3ins and tipping the scales at more than 16 stone, Harries has the ability to play both on the wing and in the centre.
He is the second summer signing to be confirmed by Cardiff Blues following yesterday’s announcement that the region had swooped for destructive tight-head Dmitri Arhip.
Head coach John Mulvihill was delighted to secure the services of Harries and believes he will play a vital role throughout the season.
Mulvihill said: “Jason is a big, powerful back three player and will complement our style of play going forward.
“He has an extremely high work rate and puts himself into good positions on the field in both attack and defence.
“He had an impressive previous with the Wales Sevens program and we are looking for him to transfer this form into our environment.”