The midfielder will remain at Vicarage Road until 2nd January.
West Brom retain the right to recall Thorne after 28 days, but hope such a situation can be avoided.
The intention is for the 20-year-old to get important minutes under his belt with Watford.
He has only recently returned to fitness from a long-term knee injury and needs game time in order to rediscover full match sharpness.
Gianfranco Zola is prepared to offer him those opportunities with the Hornets and he could make his Watford bow against Middlesbrough on Saturday.
Thorne should have no problem settling into new surroundings, with the Baggies academy graduate having previously taken in two loan spells at Portsmouth and one at Peterborough.