The Madrid-based outfit have been scouring Europe for much of the summer as it became apparent that a new left-back would be needed following the departure of Filipe Luis to Chelsea.
The Brazilian left-back reportedly told Atleti boss Diego Simeone of his plans to join the Blues before the World Cup kicked off, although his £15.8million move to West London was not competed until July 18.
Luis represented Los Colchoneros 180 times in all competitions over a four-year period, helping the club win La Liga, the Europa League and Copa Del Rey.
Blues boss Jose Mourinho knows he has an excellent player on his hands, and Atletico’s task of finding a replacement with adequate funds is an extremely difficult proposition.
Vicente Calderon director of football Jose Luis Caminero, however, believes the club have made an outstanding acquisition and one who will offer support in both fullback positions.
He said: “Ansaldi is a player who will help us increase the level of competitiveness in the group. We are talking about a fullback who can play on both sides, with great defensive abilities and who attacks with purpose. He will support us a lot with his versatility, we are very happy with this deal.”
Ansaldi is expected to battle for a first-team spot with another left-back signing Guilherme Siqueira, as both are tasked with filling the great void left by Filipe Luis.