With both Rhys Webb and Tom Habberfield out injured, head coach Steve Tandy has been left with just Tito Tebaldi as the only fit senior scrum-half at the Liberty Stadium.
The Italian number nine, himself a new signing for the region, has started the season well with Tom Grabham in reserve.
But with just one fit out-and-out scrum half in his squad at the moment, Tandy has moved to address the imbalance by adding Mack as cover.
Rugby Operations Manager for the Ospreys, Andy Lloyd, said: "We are pleased to have been able to secure Phil's services and he will provide us with some much needed cover in a crucial position over the coming weeks.
"With Rhys already side-lined, Tom's untimely pre-season injury has left us light at scrum-half.
"With that in mind, we felt it was crucial that we acted swiftly and in Phil, we have someone who has impressed at international level and who we believe can come in at short notice and do a good job for the region. We are also grateful to the WRU for their understanding of the situation."
Mack has represented Canada on the IRB sevens circuit and has played 19 times for Canada in total. His first cap came in May 2009 and he has been heavily involved with the national team over the last few months - one of the most successful periods in their history.
He played in three of Canada's four games in the IRB Pacific Nations Cup where they finished in second place behind Fiji and ahead of Tonga, Japan and USA, winning man of the match in the win over Fiji. He also started in the Rugby World Cup Play-Off win over USA in Charleston during August that helped secure Canada's place at the finals in England in 2015, scoring a try inside the first 30 seconds.
Mack becomes the third Canadian at the Ospreys, joining compatriots Tyler Ardron and Jeff Hassler, who both arrived at the Liberty Stadium earlier this summer.