The 36-year-old will replace Luca Badoer at the Italian Grand Prix as stand-in for the injured Felipe Massa.
"It is true this has always been an ambition to do this," said Fisichella, who finished second at the Belgium GP.
"I am very grateful to Force India boss Vijay Mallya for allowing me the opportunity to drive for Ferrari."
http://news.bbc.co.uk/sport1/hi/motorsport/formula_one/8231165.stm