Muniain admits a lack of consistent minutes forced the decision.
He told Marca: "Yes, of course. I have been at Athletic for many years, practically all my life, and I arrived when I was very young and joined the first team when I was 16. Now I am 31.
"My intention and my idea was always to finish my career at Athletic. What happens is that situations arise and things in terms of sport, in particular, change, so one feels obliged to rethink things, to think about them and to make decisions. I would be lying if I told you that it was an easy decision. For me it was very difficult to make the decision to leave.
"I thought about it a lot, I thought about it a lot. I spoke about it with people very close to me, but I think that in the end it was the best decision I could make, especially for the sporting aspect. I have lived a year in which I have not enjoyed many minutes or as many as I felt prepared to play and well, I felt that in some way I was prolonging my contract with the club of my life, the dynamic tended to continue.
"That's why I made this decision, with the highest possible ambition, because I feel very good and because I want to continue enjoying football and competing and feeling important."
Currently a free agent, Muniain is eyeing a move to South America, where he has been linked with Argentine giants River Plate.