Ji-soo joined Brentford B from K League Two side Seongnam in June 2023.
"Ji-soo is another example of the project here,” said Brentford B head coach Neil MacFarlane.
“Since I have been here, we have seen a number of players make the jump from the B team to the first team and this is another example of that."
Winterbottom was snapped up from Liverpool in 2021 and spent last season on-loan Welling United.
“Ben has been a fantastic servant to the B team,” said MacFarlane to the club's website.
“This season, featuring again for the B team as well as a loan move, has now led to this promotion to the first team.
“He deserves this opportunity for his high professionalism and consistency of performances throughout his time at the club.
“He is a very reliable young goalkeeper, and this is another good step in his career.”