The 56-year-old star took to Instagram to share an emotional post with her 860k followers, confirming her diagnosis.
She captioned the post simply, writing: 'Just when you thought...' alongside a note which detailed what the news had meant to her, with her signature at the end.
'1 in 8 women get breast cancer. Today, I'm the one,' she said in the image.
'The good news is that I have the most glorious group of supportive and caring family and friends, and fantastic insurance through my union.
'The bad news is that not all woman are so lucky, so let's fight all cancers and make universal health care a reality.'
Fans rushed to send messages of consolation to Julia and lift her spirits, with one saying: 'You will beat this and get through it. You are in my prayers.'
Another added: 'Sending you love, encouragement and healing, Intending a speedy recovery' while another simply put: 'You got this 💪 Sending love and light your way!'
'Wish you all the best, a fast and full recovery ❤️' commented a third fan.
Others shared their own stories of how they had also been diagnosed with cancer, and how they plan to 'beat it'.
'I am also a "one". We are going to beat the hell out of this thing. Just you watch' wrote one fan, while another said: 'You are strong👍 You can do it fight and win the battle. I survived and did it so can you 🙏💙It is not Impossible, It's I am Possible🦋💚💜'
Mum-of-two Julia is married to Brad Hall, who she tied the knot with 30 years ago. The star posted the below image on Instagram in June to celebrate their anniversary.
The actress - who is much-loved from her Seinfeld days from 1990 to 1998 - reportedly received her cancer diagnosis a day after winning her sixth Emmy for the role in the HBO comedy, Veep.