The impressive Belgian youngster was a hit at hometown club Zulte Waregem on loan over the past two seasons but now steps up to a major European league for the first time.
Brother of world-beater Eden, Thorgan is expected to become a real star and has already received his first full cap for Belgium despite missing out on their World Cup squad.
The 21-year-old is keen show both Borussia and Chelsea what he can do this season in order to further his career.
“First of all I am happy that I can now play for Borussia. I do not know too much about the club, but I hope that I will soon learn more about it.
“I know that (Germany international) Marco Reus has played here and of course I also know that my fellow countryman Filip Daems (plays there). Everything else I will certainly learn in the coming time.
“I now must also get to know my new team, so it’s perhaps a little too early to talk about it.
“I’ll try to do my best and to attend as many games as possible. That’s the most important thing for me.”