The 26-year-old back, who can play centre or wing, joins after three years starring for Wales on the IRB Sevens circuit.
Harries has spent the majority of his club career at Welsh Premiership outfit Carmarthen Quins where he came through the junior ranks.
Away from his international Sevens duties, Harries earned a reputation as one of the most consistent performers in the Welsh Premiership, with Carmarthen regularly competing for a top three play-off place in recent seasons.
The former Wales Under 18s and 20s internationalist also helped Carmarthen Quins reach the 2013 SWALEC Cup final where they were beaten by near neighbours Llanelli.
On his move to London Scottish, Harries said: “I am thrilled to have joined Scottish and look forward to this new and exciting chapter in my career.
“Playing for Wales on the Sevens circuit was a fantastic experience but I needed a new challenge and London Scottish is the right move for me.
“My time at Carmarthen Quins was memorable and I cannot thank the club enough for developing me as a player before I made my first team debut for them when I was just 17.”
He added: “London Scottish did really well to make the play-offs last year. Hopefully we can go one step further next season and make the play-off final.”
London Scottish head coach, Peter Richards, said: “I think James is a real find and will provide great competition in areas which we needed to strengthen.
“He is a hugely versatile back, a big rangy guy who carries the ball well. I’m looking forward to seeing how he shapes up when pre-season starts next week.”
