Harrington is a former assistant strength and conditioning coach at Super 14 finalists the New South Wales Waratahs and from 2004 spent three years as strength and conditioning coach with Leinster. Born in Sydney, the 34-year-old assisted Ireland in their build-up to last year's World Cup.
He said: "It's a real thrill to be joining the Scarlets at such a pivotal time in their history. The new player signings, the new coaching team and the new stadium show that the region plans a big future."
Head Performance Coach Nigel Davies said: "It has been great to bring Brad into the programme this week, he has fitted in well immediately and brings complementary skills to our coaching set-up. He has a fresh approach, lots of enthusiasm and a wealth of experience from his backgrounds in the Aussie rules game, rugby league and union.
"We are looking to intensify the conditioning area and now with two experienced fitness coaches to cover the ground we can look even more closely at specifics with individual players.
"On top of Brad and Ryan's work we are keen to offer some opportunities to sports science graduates and students interested in this area of coaching to get first-hand experience of what we are doing. Things are moving really well in training and we're all looking forward to the new season."
Latest reports from Australia indicate that the Scarlets can expect defence coach John Muggleton to arrive next week. Muggleton is a highly experienced former defence coach of Australia with whom he spent 10 years. He helped them to World Cup glory in 1999 and to victory over the Lions in 2001. Born in Sydney, the 48-year-old joins the Scarlets on a two-year contract.
The Scarlets' Wales internationals will be joining the camp on Monday (July 21), and new No8 David Lyons is expected in Llanelli the following week.