Sao Paulo goalkeeper Rogerio Ceni was also on target in a pulsating game in Tokyo the Brazilian club never seriously looked like losing despite Ittihad's brave resistance.
European champions Liverpool take on Costa Rica's Deportivo Saprissa in Thursday's second semi-final in Yokohama. The final takes place in Yokohama on Sunday.
Amoroso gave Sao Paulo the lead in the 16th minute finding space for a right-foot shot which took a deflection and trickled past goalkeeper Mabrouk Zaid.
Ittihad equalised in the 33rd minute when captain Mohammed Noor bundled in the loose ball after Sao Paulo goalkeeper Rogerio had parried Mohammed Kallon's initial effort.
But Amoroso restored Sao Paulo's advantage two minutes into the second half tucking in Real Madrid-bound Cicinho's low cross from point-blank range.
Rogerio scored from the penalty spot in the 57th minute to make it 3-1 and give the goalkeeper his 53rd goal since his first-team debut for Sao Paulo in 1993.
Defender Hamad Al Montashari pulled one back for Al Ittihad with a 68th-minute header but the Asian champions never quite had enough firepower to break down Sao Paulo for a third time.
Japan has proved a happy hunting ground for Sao Paulo who won the World Club Cup - the tournament's forerunner involving the champions of Europe and South America - in 1992 and 1993.