The Switzerland-born midfielder, 19, joined the Saints’ academy in 2018 from Geneva-based facet Servette and went on to make three first-team appearances.
A press release on Southampton’s web site learn: “The membership want to thank Alex for his efforts throughout his time as a Southampton participant and need him nicely for the long run.”
Jankewitz is hoping to kickstart his profession with Younger Boys, who at the moment are managed by former Huddersfield and Schalke boss David Wagner.
“The transfer to YB is strictly the proper step for me to develop at a excessive degree,” Jankewitz mentioned on his new membership’s web site.
“There are quite a few examples from the previous couple of years of gamers who’ve made nice progress at YB in a comparable state of affairs.
“I’m actually wanting ahead to taking part in nationally and internationally with my new team-mates.”
Southampton, who begin the brand new Premier League season at Everton on 14 August, have been linked with a summer season transfer for Blackburn striker Adam Armstrong.