The batsman will fly to the city in south Bangladesh once his visa is approved over the next 24 hours.
Hain, who is the Bears all-time leading T20 run scorer with 2,341 runs and 17 half-centuries, will be returning to the subcontinent for a second time after being selected to play for the Rangpur Rangers in 2017.
The 26-year-old will be looking to gain more short form experience after featuring for England Lions and Manchester Originals during the inaugural season of The Hundred.
"I'm incredibly excited to return to Bangladesh and test myself against some of the worlds best players," said Hain.
"There is no substitute for competitive cricket and, as players, we're encouraged to play and improve as much as possible around the world. I really enjoyed my first experience in Bangladesh and I can't wait to get out there and get going."
Captained by Bangladesh international all-rounder Shakib Al Hasan, Barishal have picked up one win from their opening three games.
The side, which also features Chris Gayle and Dwayne Bravo, return to action on Saturday (6.30am BST) and Hain hopes he'll be available for selection.
"I may have to quarantine for a short time on arrival, but hopefully my visa arrives and I can get there as soon as possible," the right-hander concluded.
"It's always great to have a fellow Bear around and I'm sure we'll try to use this invaluable experience to help the Bears win the Vitality Blast in 2022."