Olivia Munn Welcomes Second Child with John Mulaney Despite Cancer Diagnosis

Olivia Munn and John Mulaney officially have double trouble in their home ... welcoming kid #2 earlier this month via surrogate -- this months after she revealed she had breast cancer.

Munn shared the news to Instagram Sunday ... posting several pics of herself and Mulaney with their newborn -- Méi June Mulaney, a girl, born Saturday, September 14.

OM explains the two used a surrogate ... adding she had very strong feelings about not carrying her own baby this time around -- but, their surrogate showed her grace and helped her put her mind at ease.

Munn ends her post by sharing her daughter's cute nickname -- "Plum" -- 'cause that's what Méi means in Chinese.

John and Olivia welcomed their first kiddo -- Malcolm -- back in 2022 ... months after Mulaney divorced his wife and finished a stint in rehab for cocaine addiction.

They lived happily together since ... though back in March, Olivia revealed she's suffering from breast cancer -- which she was first diagnosed with in 2023.

She's already undergone a double mastectomy and several other surgeries -- including a hysterectomy, hence needing a surrogate to carry the child.