The 25-year-old Frenchman reunites with former Tangerines boss Ian Holloway.
Grandin was released at the end of last season, following a loan spell at Nice.
"I had a good year in the Premier League with the gaffer," he said.
"It's very nice [to be back in the Premier League] because every game is a new challenge.
"In the French league or in the Spanish league, you have six strong teams, but in the Premier League I think all the teams are very strong."