The pair made their feelings known at a press conference in Rome, where they have been promoting the Italian release of the blockbuster Minority Report.
"If Bush, as I believe, has reliable information on the fact that Saddam Hussein is making weapons of mass destruction, I cannot not support the policies of his government," Spielberg said.
The director added that those policies were "solid and rooted in reality".
Cruise also spoke out in support of the US president.
"Personally, I don't have all the information President Bush has," said the star.
"But I believe Saddam has committed many crimes against humanity and his own people."
Opposition
The pair travelled to Rome from Paris, where the sci-fi blockbuster also received its French première this week.
Cruise and Spielberg are among the first celebrities to speak out in favour of action against Iraq.
A delegation of UK stars set out their opposition to an American attack on the Middle East state last week, delivering an open letter to Downing Street.
More than 100 actors, writers, musicians and playwrights said in the letter that attacks on Iraq qould be "unjustifiable" and have "potentially horrific ramifications".
"It would open a Pandora's box which may be impossible to close," the letter added.
Playwright Harold Pinter, film director Ken Loach and musician Brian Eno were among those who signed the letter, also supported by actress Catherine McCormack, comedian Mark Lamar, and members of Blur and Massive Attack.
Actresses Jemma Redgrave, Frances de la Tour and Susannah Harker were among the delegation who delivered it to Downing Street.