The reigning European champions were drawn alongside Italy, Croatia and Ireland for the group stages of next year's competition.
And despite being favourites to top the group and also to retain their title, Casillas insists it would be a mistake to be over confident.
"The European Championship 2012 draw has paired us with Italy, Ireland and Croatia. We must never be too confident, the respect towards all three teams has to be at its maximum," the Real Madrid goalkeeper stated on his official Facebook page.
"We know that every game we will be defending the title won four years ago. It will be a great tournament, there are very good teams who will give us some excellent matches."
Spain's first opponents will be Italy, whom they meet on June 10.