The 12-minute footage was shot in October 2012 before Manning was sentenced to 35 years in detention for leaking classified US documents. In the video, Lady Gaga is filming Assange at the Ecuadorian embassy in London and asks him questions about Chelsea Manning's fate.
"Do you feel badly that [Manning] is in this situation?" Gaga asked.
"I feel that we have a duty to fight, to draw attention to his plight. If she has truly done what she has alleged to have then she is a hero, there is no doubt that she is a hero," Assange replied.
Manning will be released from prison this week, after her sentence was commuted by President Barack Obama, and it turns out that she will remain on active duty in the US Army once she is free.