Spieth, who was playing in the championship round of The Open Championship (AKA the British Open), one of golf's most prestigious tournaments, made bogey on the 17th hole.
Perhaps coincidentally, the sports apparel company's stock dipped from its session high of $89.47 at around 1:34 p.m. ET to as low as $88.79 at about 1:47 p.m. ET after Spieth's shot.
With a market cap of about $19.2 billion and roughly 215.7 million shares outstanding, the dip translates into a loss of about $120 million.
The company's stock had traded higher prior to Spieth's shot on 17th.
Spieth's successes on the links this year have been a boon to Under Armour, which bet on the young golfer to help propel its brand.
Despite the dip, Under Armour shares remained up about 2 percent in afternoon trading while Spieth remained tied for fourth place.
The company was not immediately available to comment on Spieth's performance in the tournament.