The 19-year-old, who scored 20 goals while on loan at Coventry City last season, was called up for England’s under-20 squad on Thursday ahead of their friendly against Brazil next month.
And in the evening, he put pen to paper on a deal which will keep him with his boyhood Club until 2020.
Armstrong, who made his Premier League debut as a 17-year-old against Fulham in March 2014, said: “I’m over the moon.
“I’ve been working hard on the training pitch and I did well last season, so to get the reward is massive for me and I think it’s the next step.
“I went on loan last season and for me to come back here and extend my contract is brilliant for me and for my family.
“I’ll keep working hard every day and progress.”