Mkhitaryan, 31, joined the Italians at the start of the season after a disappointing 18-month spell at Premier League club Arsenal following a swap deal in January 2018 which saw Chile forward Alexis Sanchez move to Manchester United.
"I'm very pleased," Mkhitaryan told Roma's website.
I'm happy to extend my stay here because I still have things that I haven't achieved yet that I want to achieve. I'm looking forward to it.
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The Armenia international has scored six goals and provided four assists in all competitions for the Rome club. His loan deal was set to expire on Tuesday.
"We are happy that Henrikh has decided to stay with us, and help us continue to grow," Roma CEO Guido Fienga said.
"The club's targets have not changed and we always want to have players at our club, like Micki, who show their desire, professionalism and determination, who represent our club and our fans in the right way."
Roma are fifth in the standings with 48 points and on track to qualify for the Europa League. They are nine points behind Atalanta who occupy the fourth and final Champions League spot with 10 games to play.