Sarah Bowmar, an ambassador for the brand, filmed her husband Josh Bowmar killing the animal with the homemade weapon, which was also fitted with a GoPro camera, providing a "spear eye view".
Under Armour sponsors Bowmar Fitness, a company run by the couple that provides fitness plans and nutritional supplements.
The video was uploaded to YouTube in June. It appears to have been removed from the pair’s hunting channel, Bowmar Bowhunting, but can be seen elsewhere online.
Under Armour had condemned the killing, but had previously declined to cut its ties with Sarah, while insisting that Josh did not have a relationship with the company.
But it finally caved in, after a petition on Change.org gathered more than 4,500 signatures.
A spokesman for the brand told The Daily Mirror: "Under Armour is ending its relationship with Sarah Bowmar, the wife of Josh Bowmar. The method used to harvest this animal was reckless and we do not condone it.
"Under Armour is dedicated to the hunting community and supports hunting that is conducted in compliance with applicable federal, state and local laws and hunting practices that ensure a responsible and safe harvest of the animal."
However, the brand’s move was not without controversy in the hunting community.
And Sarah Bowmar took to social media to defend the ethics of her lifestyle.
Authorities in Alberta have also said that Josh Bowmar may be charged for the killing. Hunting with spears is currently legal in the province, but may be outlawed this year.