Cassano and Pazzini complete Milan swap
Negotiations between the two Milan clubs had been ongoing but a deal has now been reached, although the length of Pazzini's contract and financial details were not immediately available.
Well-travelled Cassano, who missed much of last season after a heart problem, has penned a two-year deal with Inter.
"Finally I am at the team I have always supported," the former AS Roma and Real Madrid player said in a statement.
Cassano's departure from AC Milan comes as a surprise given he had revived his career there since joining from Sampdoria 18 months ago.
He became firmly established in the Italy lineup unlike Pazzini, who lost his place after a poor season with Inter.
Milan, who cashed in on tall forward Zlatan Ibrahimovic when they sold the Swede to big-spending Paris Saint-Germain, have opted for Pazzini as he offers more physical presence up front than the tricky Cassano.