The sought-after 26-year-old, who spent a brief loan spell with the Pilgrims in December 2018 before sustaining an injury on his debut at Chorley, has put pen-to-paper to become United's second signing of the summer following Luke Shiels' arrival from Alfreton Town.
Thewlis, who began his career at Scunthorpe United and has also played for Gainsborough Trinity in between two spells with Harrogate, can operate in a variety of forward positions.
Speaking on the permanent signing of Thewlis, United boss Craig Elliott told bufc.co.uk: "We are delighted that Jordan has chosen to join us after leaving Harrogate.
"I was really looking forward to working with him until injury cruelly cut short his debut and loan spell but he was flying for Harrogate towards the end of the season and he should be a great addition after dropping down a league."
Updated 17:06 - 13 May 2019 by Craig Singleton