20 minutes into the match, the 18-year-old Bellingham was nicely in front for the first time, he received the ball in the penalty area and rolled it nicely between goalkeeper Ersin Destanoglu's legs.
Just before the referee blew for a half-time break, the young Englishman arrived again.
After a nice dribble from the left wing, he played the Norwegian star Erling Braut Haaland who put 2-0.
The goal was the Norwegian's 21st in just 17 Champions League matches.
No player has previously been close to reaching 21 goals in so few games.
In extra time, another talent, 19-year-old Ansgar Knauff, was close to scoring his first Champions League goal but the shot went into the crossbar.
Instead, Besiktas reduced by Francisco Montero who nodded in the newly acquired Miralem Pjanic's free kick.
2-1 was also the result.
Dortmund, who last season went against Manchester City in the quarter-finals, got the best possible start in Group C, where Ajax and the Lisbon team Sporting can also be found.