Moore tells The Associated Press in an interview that he considers Pfc. Bradley Manning a courageous patriot for exposing what Moore called "war crimes."
The Oscar-winning filmmaker said Thursday he will contribute to a legal defense fund for the 22-year-old former intelligence analyst, who faces up to 52 years in prison.
Manning is charged with leaking video of a U.S. Apache helicopter attack that killed 11, including a Reuters news photographer and his driver.
Video: 2007 U.S. Apache Helicopter Attack
Investigators call Manning a person of interest in the release of 77,000 Afghanistan war logs that WikiLeaks published online in July.
WikiLeaks: We Won't Be Threatened by Pentagon
WikiLeaks Preparing to Release More Afghan Files
Gov't Wants Wikileaks to Hold Back 15,000 Docs
Mullen: Afghans Outed by WikiLeaks Are Unsafe
Pentagon: WikiLeaks Endangers Soldiers, Afghans