Cassani has joined on loan from Fiorentina with an agreement in place to make the deal permanent if he can impress.
The 29-year-old has 10 caps for Italy and divided the bulk of his career between Verona and Palermo before moving to Fiorentina last year.
The veteran Portanova, 34, has joined on a permanent deal from Bologna, where he played more than 100 games since joining in 2009 from Siena.
Both deals were confirmed on Genoa's website.
Meanwhile, the club also announced that 20-year-old defender Mario Sampirisi has been loaned to Chievo while young forward Leonardo Melazzi has returned to Danubio in his native Uruguay, also on loan.