The former Manchester United and Inter Milan defender has signed a one-year deal at Portman Road.
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"Ashley has had a fantastic career and his achievements speak for themselves," Ipswich boss Kieran McKenna told the club website.
"He has maintained an extremely high level of professionalism and competitiveness through his career and he is still playing at a very good level.
"We feel his leadership and experience will be valuable to the group this season and he is very hungry to contribute on and off the pitch."