During his trial at Arsenal in the summer of 2010, Miyaichi impressed the Arsenal manager, Arsène Wenger, signing a professional contract on 31 January 2011. Wenger said: "We’re delighted that Ryo Miyaichi has joined us. He trialled with us in the summer and has raw ability which has attracted many clubs around the world".
Just after he signed for Arsenal, it was announced that he was set to join Feyenoord on loan. On 5 February, in his first appearance for Feyenoord, he played the full 90 minutes against Vitesse Arnhem in a 1–1 draw, and was named man of the match. Miyaichi then played against Heracles Almelo where he scored the opening goal and created the second in a 2–1 league win for Feyenoord. On 17 April 2011, Miyaichi scored two goals and contributed two assists in Feyenoord's 6–1 victory over Willem II, leading a revival after Feyenoord had conceded an early goal.
On 9 August 2011, Miyaichi was granted a work permit to play for Arsenal on the grounds of being an "exceptional talent" in view of evidence supplied by Wenger and the Japan Football Association, and was deemed free to represent Arsenal during the 2011–12 season. He was also included in Arsenal's 23-man Champions League squad for the fixture against Udinese. On 23 August 2011, Miyaichi featured in the Arsenal Reserves against Wigan Athletic Reserves and scored in the 83rd minute.
On 20 September 2011, Miyaichi made his first team debut against League Two side Shrewsbury Town in the League Cup third round as a 71st-minute substitute. He also came on again in the League Cup as a substitute as Arsenal went on to win the match 2–1 against Bolton Wanderers.
During the reserve match against Fulham, on 7 November 2011, Miyaichi lasted 34 minutes before being forced off with an ankle injury, which would cause him to be out for weeks. Despite being in the first team, Miyaichi admitted he still has a lot to learn.