The 26-year-old is scheduled to be a restricted free agent on July 1.
Ersson spent each of the last four seasons with the Philadelphia Flyers, who selected the Swedish netminder with the 143rd pick of the 2018 NHL draft.
He was dealt to Toronto in mid-June alongside a 2026 third-round pick and defenceman Emil Andrae in exchange for Simon Benoit and Joseph Woll.
Through 143 total games of NHL experience, all with Philadelphia, Ersson holds a .884 save percentage and 3.01 GAA. He now joins the Senators on the heels of a 2025-26 season in which he made 29 appearances (14 starts) and went 14-11-5 while posting a .870 save percentage and 3.12 GAA.