Writing on his official website, Wendy Dio said the former Rainbow and Black Sabbath singer was in the early stages of the illness.
Mrs Dio added he was starting treatment immediately and would be back on stage after "he kills this dragon".
Dio, 67, had to cancel his band's European tour last week after he became unwell and was taken to hospital.
"Ronnie has been diagnosed with the early stages of stomach cancer. We are starting treatment immediately at the Mayo Clinic," Mrs Dio said.
"After he kills this dragon, Ronnie will be back on stage, where he belongs, doing what he loves best, performing for his fans."
Mrs Dio also thanked fans for their well wishes, adding: "This has really helped to keep his spirit up."
Dio first achieved recognition singing with his own band Elf before joining Rainbow in 1975.
He later replaced Ozzy Osbourne in Black Sabbath in 1979 for three years.
He is currently working on a project with former Black Sabbath bandmates Tony Iommi, Geezer Butler and Vinny Appice, under the name Heaven & Hell.
http://news.bbc.co.uk/2/hi/entertainment/8380305.stm
