The Gunners announced the move on their official website just as the deadline approached, saying they had paid an undisclosed fee for the 26-year-old.
The Spain left-back arrives at the Emirates Stadium at a perfect time for manager Arsene Wenger, who last night lost Kieran Gibbs for three weeks with a thigh injury.
"We are delighted that Nacho Monreal has agreed to join us. We have been monitoring him for some time now and are really pleased that we've been able to agree this move today," Wenger said.Check www.arsenalinterest..com for the details.