The Saracens Academy member, 21, played in the Under-20 World Championship in Georgia in 2017-18 and in the age group’s Six Nations.
He joins TJs from the Toronto Arrows in Canada, where he was on loan from Sarries for the Major League Rugby season.
During the 2017-18 season he was loaned out to the Bedford Blues, where he scored six tries in for the Championship side.
He also scored a memorable solo try for Saracens against Harlequins in the Anglo-Welsh Cup under captain Ben Earl, the England squad player who went to Tonbridge School.
Jack, who is 6ft 4in tall and weighs 240lb, said: “At this stage in my career moving to an ambitious, well organised club like TJs is perfect.
“It will give me the opportunity to play a full season at a high standard and will definitely help my development, and hopefully I can play a key role in securing a league title for the club.”
Head Coach Peter Dankert said: “As a club we are very ambitious, and with the support from our sponsors such as Ridgecrest Cleaning, we are now in a position to attract players of Jack’s quality, which will make a great difference to the strength of our squad.
“We are all very excited for the coming season and want to welcome rugby fans from across Kent to come to Tonbridge to see stars like Jack in action.”
TJs start their league campaign at home Leicester Lions on September 7.
