Hitzlsperger lost the Stuttgart captaincy this season before coach Markus Babbel was fired in December with ex-Tottenham Hotspur coach Christian Gross taking over and steering the German side to the UEFA Champions League's last 16.
Stuttgart are up to 13th in the league and are unbeaten after six games under Gross, but former Aston Villa midfielder Hitzlsperger is heading to Rome under advice from national coach Joachim Loew.
Gross had mainly kept Hitzlsperger on the bench recently and with South Africa 2010 less than five months away, Loew advised him to switch clubs to make sure he has regular football, with places in the Germany squad up for grabs.
"I am looking forward to the new challenge," the 27-year-old said. "I had a good time in Stuttgart and I will always have good memories."
He was part of the Stuttgart side that won the German title in 2007, but spent the weekend in Rome finalising his contract and will train with his new team on Tuesday. Italian sports paper Gazetta dello Sport report the deal was worth €3m, but Hitzlsperger will not have an easy time in the Italian capital with his new club also fighting against relegation.
Hitzlsperger has won 51 caps for Germany and made more than 125 Bundesliga appearances, scoring 20 goals since joining Stuttgart from Villa in 2005.