Biggar joined Saints from Ospreys in 2018, making 69 appearances for the club and guiding them to two fourth-placed finishes.
The flyhalf, whose final game came in last month's 40-36 win over Wasps, had previously said he would leave at the end of the 2022-23 season.
"I know this is a squad which can challenge for major honours this season, so this has been a very difficult decision for me to make," Biggar said in a statement.
"I move on with a heavy heart and I am grateful to the Club for their understanding throughout the process.
"I never envisaged leaving Saints midway through the season, but this opportunity came around very quickly, and I made my decision with a long-term view in mind for my family."
Biggar has 103 caps for Wales, winning the Six Nations title in 2013 and 2021. He also captained Wales during their 2022 Six Nations campaign.
Saints' chief executive Mark Darbon said Biggar left with the club's "very best wishes," adding that the Welshman's move was brought forward because Toulon's offer was contingent on him moving immediately.
Saints are fifth in the standings and next face Exeter later on Friday.
