The experienced Treeby will be with the Stormers for the next four months as they look to build on what has been an encouraging start to their 2017 Super Rugby campaign.
The 28-year-old former Highlanders man comes in to give the Cape side options with international centres Damian de Allende, Juan de Jongh and Huw Jones all ruled out for the long term.
Western Province Director of Rugby Gert Smal says it was important to bring in an established player as cover.
Smal says: “Having lost Damian de Allende, Juan de Jongh and Huw Jones to injury this year, we needed some experience in midfield and I am sure that Shaun will have a positive impact here in his time with us.”
WP Rugby Group CEO Paul Zacks says he is excited to see what value Treeby can add to the squad.
Zacks says: “Shaun made his Super Rugby debut for the Highlanders in 2011 and has amassed 50 caps, so he is expected to have a big impact here at Newlands.
“He is a proven competitor and can only help some of the talented young players around him to grow.”