Town’s young stopper has signed a two-year deal to remain in Somerset until the summer of 2019.
Maddison joined the Glovers from Leicester City and played in his first professional game for the club in the Checkatrade Trophy against Portsmouth.
The 22-year-old featured regularly in the competition, helping the Green and Whites reach the quarter-finals.
In April, Maddison made his first league outing at Newport County and finished the campaign between the sticks, making a total of five appearances in Sky Bet League Two including one clean sheet.
“Jonny is a good young keeper with huge potential,” boss Darren Way said
“He was thrown in last season and had to play a major contribution to the team.
“He’s a fantastic character, works extremely hard and I’m sure he has a bright future in the game.”