Movie superstar Brad Pitt has stood by his mom despite her slamming marriage equality and campaigning against President Barack Obama.

Jane Pitt shocked the Hollywood A-lister's fans after she wrote an anti-gay marriage letter to a newspaper urging voters to back Republican nominee Mitt Romney in the US elections.

However, the Killing Me Softly star insists there is no bad blood between him and his mother, despite the odd dinner table debate.

'It's a nice, very open discussion. We grew up with a specific set of rules and challenging those rules has never been a closed door. Certainly our views can evolve,' Pitt told People magazine.

'I come from a Christian family, and (gay marriage) goes against Christian beliefs, as far as what's going to get you into heaven or not. My argument is, that may be as you believe, and it may be true in the end - I don't think so - but let your God make that call.

'In the meantime, we live in a country where everyone should be treated equally, so let's treat everyone equally.'

Pitt has previously said gay marriage is 'inevitable' and publicly backed Barack Obama's campaign to be re-elected President of the United States.

The actor and his superstar partner Angelina Jolie are well-known fighters for LGBT rights, contributing $100,000 to the campaign to defeat California’s Proposition 8 in 2008.

He played gay United States District Chief Judge Vaughn R Walker in Dustin Lance Black’s play ‘8’, who came out after ruling Prop 8 was unconstitutional.

Some have argued Pitt also played bisexual or gay in the 1994 movie Interview With A Vampire, in which he has an ‘intense’ relationship with Tom Cruise’s character Lestat.