19-year-old Jankewitz has joined the Swiss club for an undisclosed fee from Saints, and signed a four-year deal with Young Boys.
"The move to YB is exactly the right step for me to develop at a high level," the Switzerland U21 international told the club's website.
"There are numerous examples from the last few years of players who have made great progress at YB in a comparable situation.
"I'm really looking forward to playing nationally and internationally with my new teammates. "
Young Boys sporting director Christoph Spycher added: "We have followed Alexandre Jankewitz's path with great interest for years and are glad that he has now decided on YB despite a few other offers.
"Although he is only 19 years old, he has already gained a lot of experience that will be valuable for his further career. He is a very strong player who will fit in very well with us with his attitude and his will."