Thirlby has spent the last two seasons with Penzance and competing on the international sevens circuit for England.
The new Gloucester recruit, who can also play at full-back, was top of the IRB's try scoring charts last season with 35 touchdowns throughout the Sevens season.
Thirlby, who toured South Africa with the full England set-up in 2000, knows a few familiar faces at Gloucester from his international sevens experience including Simon Amor, James Bailey, Peter Richards and Henry Paul.
Commenting on www.gloucesterrugby.co.uk he said: "I'd been speaking to Gloucester and Dean (Ryan) confirmed everything recently, which is great, because I really wanted to get back into the Premiership."
Thirlby scored six tries in 21 starts for Saracens, before sealing a two-year deal with Bath in 2000. After a strong first season with Gloucester's West Country neighbours, when the winger scored eight tries in 14 starts, he struggled with injury, and went on loan to Rotherham for the remainder of the 2002/03 season.
"I joined Penzance in the summer of 2003 to concentrate on getting back to my best form. I'd had a couple of injuries that had disrupted my game.
"I just wanted to get back to enjoying my rugby again, and playing for England Sevens and Penzance allowed me to do that.
"I've had a good couple of years in Cornwall and I'd like to thank them for getting me back into form, like I was when I was at Saracens and in my first year at Bath.
"It gave me back my confidence, and I got back to beating players. I was getting my sharpness back, which is what someone like myself needs."
"I wanted to give the Premiership another go, and I can't wait to get started again when pre-season starts at the end of the month."
Thirlby will join Gloucester's other new recruits, including Mike Tindall (centre from Bath), Patrice Collazo (prop from Toulouse), Peter Richards (scrum-half from Wasps), Ludovic Mercier (fly-half from Pau), Haydn Thomas (scrum-half from Exeter Chiefs) and Anthony Allen (centre from Harlequins).
Tongan wing Seti Kiole, who joined Gloucester Rugby in the Autumn of 2004, has left the Kingsholm club after his contract expired. Scrum-half Alex Page has also moved on after two seasons with Gloucester.
These two join Christo Bezuidenhout (prop), Nathan Mauger (centre), Noel Curnier (hooker), Chris Fortey (hooker), Andy Gomarsall (scrum-half), Nick Runciman (scrum-half) and Rory Teague (full-back), who have also moved on.
Rob Thirlby Profile:
Position: Wing/full back
Date of birth: 02.03.79
Height: 1.86m (6'1")
Weight: 84kg (13st 2lb)
Representative hons: England 'A', Sevens and U21s