The Black Swan star is expecting her first child with fiance Benjamin Millepied.
They have joined over 100,000 supporters of the campaign urging US President Barack Obama to support the freedom to marry for gay and lesbian couples.
Evan Wolfson, President of Freedom to Marry, said: 'Natalie Portman and her fiance, Benjamin Millepied, are in love, and believe all loving couples should be able to share in the freedom to marry without discrimination.
'That is why they are joining the more than 100,000 people on Freedom to Marry's Say, 'I Do' Open Letter calling on President to do the right think and become part of America's majority for marriage.'