The 21-year old midfielder has penned a one-year extension on his current deal which will now keep him at Tannadice until the summer of 2016.
Armstrong, who joined the Tayside club from Inverness as a 17-year old in 2009, was delighted to commit himself to the Arabs' cause.
"I am enjoying my football, scoring goals and improving as a player so signing the extension was the obvious choice," he said.
"It is such a joy being involved in our games and all the players have a similar vision to the manager as to where we can go as a unit so I look forward to the next few years being very exciting for United and our fans."
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