Thangam Debbonaire, 48, said her consultant had told her the cancer was "perfectly treatable" and that she is "expected to make a full recovery".
Ms Debbonaire said she had picked up the symptoms via a public health campaign and urged women to check their breasts regularly.
The Labour MP said she had arranged backup and support in parliament to cover her absence.
"No matter how I'm feeling, I will ensure I represent and support the people of Bristol West effectively, with the help of a first-class constituency team," she said.
Ms Debbonaire is due to start treatment at Bristol Oncology Centre on 3 July.
She received 35.7% of the vote in the constituency during May's general election taking the seat from Liberal Democrat Stephen Williams.
Bristol West's newly-elected MP has been diagnosed with breast cancer.