Last season, seven points from their final five matches ensured that Forest Green remained in League Two, a year after gaining promotion from the National League.
The Gloucestershire-based club are now moving to strengthen ahead of the new campaign, and that has resulted in 20-year-old Sanchez being given the chance to impress at The New Lawn.
Brighton Under-23 coach Simon Rusk told the club's official website: "Robert has been excellent for us and he fully deserves his chance to go out on loan and play senior football.
"We managed to keep a number of clean sheets last season, which helped us achieve promotion, and Robert was a key member of the squad that contributed towards that.
"Forest Green are getting a promising young goalkeeper and Robert will be gaining more experience on his journey within football - therefore it is a great move for all parties involved."
Sanchez is yet to make a first-team appearance in English football.