The 22-year-old has been with the Hatters since joining from Huddersfield Town on deadline day 2016.
Senior had spent nine years with the Premier League club, having joined when he was just nine years old and will wear the number three shirt at The CNG Stadium.
The Halifax-born defender has made over 20 appearances for his current side Luton, and signed a one year contract extension with the option of a further year in November 2017.
Jack will be available for selection this weekend in the FA Trophy, subject to FA approval while for sponsorship options, please email Bensadler@harrogatetownafc.com.
We would like to welcome Jack to the club and thank Luton Town for allowing the deal to be completed.