Treeby (28) has signed with the Cape Town-based franchise after injuries to key players in the Stormers midfield.
The Stormers, who have won their first five games in the competition, and lead the African one conference, have lost Damian de Allende, Juan de Jongh and Huw Jones all to injury this year.
Treeby first played for the Highlanders in the 2011 season and was with the franchise for five seasons, playing his final game in 2015, the year the Highlanders won the title.
His career with the Highlanders was blighted by injuries, although he did play 50 games for the side.
He was not picked up for the 2016 Super Rugby season and has continued to play for the Wellington provincial side.
A combative midfielder with good defence, he is a quality player, although his relatively small stature sometimes counted against him.