Freya Bevan, from Neath, was diagnosed with a brain tumour last April and needs Proton Beam Therapy - a treatment that will not become available in the UK for three years.
The funds raised will help Freya travel to Jacksonville Proton Centre in Florida for treatment, and Ospreys chief executive Andrew Hore says the club will be supporting her in any way they can.
He said: "Freya is a wonderful little girl who remains cheerful and positive despite everything she has been through, and continues to go through.
"She is an example to us all. We are honoured to be able to use our high profile to support her parents, John Paul and Katherine, in their efforts to raise the money needed to potentially fund this treatment privately."
Freya was only 19 months old when she was diagnosed and has since gone on to suffer a mini-stroke and two seizures during her treatment.
She is being treated at University Hospital Wales and has undergone two major brain surgeries, one lasting seven hours, and desperately needs to get to America to further her treatment.
The Freya Bevan Trust was established in support of Freya aiming to raise support and funds in order to support her and her family.
Anyone looking to lend their support to the campaign can visit Facebook (Freya-Bevan-Fund) or Twitter (@FreyaBevan_Fund) or donate here.
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