The former Reds favourite, 34, moved up from the academy to assist Stendel alongside Christopher Stern following Andreas Winkler's departure to Huddersfield Town in January.
Bolton-upon-Dearne-raised Tonge has proved a popular figure with players during his time working alongside Stendel, playing his part in orchestrating the club's strong second half to the campaign, which culminated in promotion back to the Championship at the end of last season.
The boyhood Reds fan spoke about his desire to continue his coaching education alongside Stendel at Oakwell and a deal has now been finalised for him to remain in his current position at the club.
On his new deal, Tonge - promoted as a player to the second-tier in a joyous 2005-06 season under Andy Ritchie - said: "I am buzzing. It was always going to get done, the gaffer told me he was happy with what we were doing and how we were working together, so after a quick holiday at the end of the season I was back in the office and able to get this signed, as well as working on plenty of different things for next season.“I love the club, it was an honour to play for Barnsley and to still be involved now on the coaching staff is what I always wanted when I finished my playing days. We’ve felt a massive high from promotion and we have rightfully enjoyed it, now it is time to continue the progress and see where next season takes us.”