The 22-year-old impressed director of rugby Graham Dawe during a week’s trial with the club at the start of July.
However, the former Lyon player was not available to move to Plymouth until this week.
He arrived in the city last night on the late cross-Channel ferry and he will train with his new team-mates today ahead of their trip to Hull Ionians on Saturday.
“He came for a week’s trial at the beginning of July,” said Dawe.
“However, he was not available for the start of the season as he had a work contract to complete.
“But he has followed our training programme to the letter and we’ll see how he goes in the next couple of weeks.”
Dawe added: “He’s a very fit and fast individual. We want to utilise everything he brings to the party and, hopefully, we can bring some structure to his game.
“He will certainly add competition to the squad.
“He is primarily a flanker, but he can also play number eight.”
Sebastien started his rugby career as a junior with Saint Genis Laval before joining Lyon. He attended final selection trials for the France under-17 team.
He was working with Renault Trucks in Lyon before moving to Plymouth.
Sebastien’s arrival at Brickfields comes at just the right time as the club are set to lose Royal Navy flankers Jarrad Hayler and Seti Raumakita for a month due to the Defence World Cup.
Hayler has started Albion’s opening two matches and looks a real find for Dawe.
The 20-year-old had previously been playing league rugby for Havant.
“Jarrad will certainly be away for at least three weeks, but it depends on how far the Royal Navy go in the competition,” said Dawe.
“It was something we knew about before the season start.
“Jarrad’s a good athlete, who is young and enthusiastic. He’s strong in the tackle, a strong ball carrier and makes up ground quickly.”
Meanwhile, former development back-row forward Jake Childs is training with Albion.
Childs made two first team appearances for Albion during the 2006/07 season before going on to play for Launceston and Taunton.
“He’s just training with us,” said Dawe. “He’s just come back from America but he’s looking strong and fit and we’ll just see how he goes, but we know it is going to be very competitive in the back-row now.”
It is believed Launceston are also interested in re-signing Childs, who played alongside Cornish All Blacks coach Mike Lewis at Albion.
Meanwhile, Albion have agreed to allow winger Jon Dawe to play for Launceston this weekend as he continues his comeback from a knee injury.
Cornwall player Dawe moved to Brickfields in the summer from Polson Bridge.