The Japanese international, who has been out on loan for the last three years, has made four appearances in all competitions for the Gunners this year.
Miyaichi, 20, is now relishing his time around the senior squad at the Emirates Stadium.
"Having been given a few chances this season it's been a great experience," he told the club's official website.
"With the team in really good form there's also a great atmosphere in training, in which I have a lot to gain, so I'm really enjoying every single day."
Arsenal are back in Premier League action on Saturday when they entertain Norwich City.