The defender has joined Real Sociedad after six-and-a-half years at the Emirates.
In a post on Instagram, Monreal wrote: "31st January, 2013. Malaga. I wake up, check my phone and have 2 calls from Santi Cazorla. I call him and he asks me if I want to play for Arsenal. 10 hours later I was an Arsenal player. Sometimes Football is simple.
"Different country, city, language, football style, team mates, where it's not the best conditions for a shy person as me, but I knew I have to be there!! Almost 7 years later it's time to say goodbye, it hasnt been an easy decision, but thinking of my family and my future it feels that it is the right decision.. I would like to say thank you to all my team mates, staff, and all the people who work for Arsenal and especially to the fans for all the respect and love they have always shown me.
"I'll always remember you. THANKS.
"P.S Now it's time to win the North London Derby."