The 36-year-old Argentinian was a free agent after leaving Inter at the end of last season and, after rejecting an offer from Serie A club Sampdoria, he has opted to join the reigning Swiss champions.
Basle president Bernhard Heusler told the club's official website: "In Walter Samuel Basle have signed a very experienced player with a great personality to boot.
"Once again, this is a transfer that proves how attractive Basle and the Swiss league are becoming."
Samuel, who made 172 league appearances over his nine years with Inter, added: "I am very happy and looking forward to the new challenge and adventure offered by Basle."
The new Swiss Super League season is already under way, with Basle clinching a 2-1 victory in Saturday's opening-day trip to Aarau.