Alonso has brought an end to his two-year stay at the Reebok Stadium by turning down the offer of a new deal and signing for the Viola, who finished the campaign fourth in Italy and will play in European competition next season.
The Spanish full-back, 22, has spent the last three seasons with Bolton, after signing from Real Madrid in the summer of 2010.
He made a total of 46 appearances for Wanderers, scoring five goals.
A statement on the club's official website read: "Bolton Wanderers would like to thank Marcos for his contributions during his time with the club and wish him well for the future."
- Sports & Recreation