The 23-year-old revealed the allure of joining los Blancos has proved too strong to resist as he finalises a move to the Spanish capital
The Spain Under-21 international is on the verge of swapping the Anoeta for the Santiago Bernabeu this summer after the Blancos confirmed they had triggered the youngster's €30 million (£25.9m) release clause.
And, while Illarramendi stressed it was not an easy decision to leave San Sebastian after 12 years at the club, the 23-year-old says he could not resist the allure of the Spanish giants.
He told a press conference: "I became aware of the interest whilst on holiday, there was contact, the only ones I spoke to were my agents.
"I spoke with my people, we considered it and I thought this is a unique and irresistible offer. I feel the colours of La Real and will always be a fan, I struggled a bit to make this decision, but it is a good decision.
"I have been here 12 years and the decision was not easy, I had a hard time deciding. I want to give thanks to the directors, the coaches in the youth team, my team-mates, I wish them all well."
Illarramendi will become the fourth summer recruit to join the capital club following the arrivals of Carlos Casemiro, Isco and Daniel Carvajal.