The firm - which sponsors British tennis star Andy Murray - forecast revenue for this year would be below analysts' estimates as well.
It also said its finance chief was standing down for personal reasons.
In the final three months of 2016, revenues rose 12% to $1.31bn, but that was the slowest growth for eight years.
Profits fell to $104.9m from $105.6m a year earlier.
Under Armour founder and chief executive Kevin Plank said on a call with investors the company needs to "become more fashionable".
For 2017, the company expects revenues to rise by between 11% and 12% to nearly $5.4bn, but this was below market forecasts of more than $6bn.
Under Armour shares sank 23% to $19.22 on the news, marking its biggest fall in nine years, and wiping out more than $3bn in market value.
The Dow Jones fell 107 points, or 0.5%, to 19,864 by close of trading on Wall Street.
The S&P 500 index dropped 2 points to 2,279, while the Nasdaq pulled back from earlier losses to rise 1 point at 5,615.
UPS shares fell 6.75% after the delivery company predicted full-year profits below market expectations, saying its results would be hit by the strength of the dollar.
Healthcare stocks were among the biggest winners after President Donald Trump said he would cut taxes and regulation in a meeting with pharmaceutical bosses.
Stocks on Wall Street closed lower, with shares in sportswear maker Under Armour plunging by a quarter after its sales and profits missed estimates.