The Harry Potter author - who recently wrote The Cuckoo's Calling under her pseudonym Robert Galbraith - revealed the "atrocities" she witnessed in state-run care facilities in Eastern Europe as she relaunches her charity Lumos, which is dedicated to finding solutions for such children.
She said: "I was standing at this cot and I just thought, 'I will take her.' It was utterly irrational but that is your human response.
"There was no earthly way I could take this baby home but that is your most powerful reaction. You think as a mother, 'I will save this baby, this one baby.'"
The mother of three has poured millions of her own money into the charity, which has already helped 7000 children, and has made visits to see its work in Moldova, the Czech Republic, Ukraine and Romania.
The aim of the charity is to liberate children from such institutions and return them to families, a goal that chimes with the experiences of her most famous character who, she said, was left in a "hostile environment" as an orphaned child.
Most of the children Lumos aims to help are not orphans but children who are in homes because of their parents' poverty or personal situations.
The author, who said she was once "as poor as it's possible to be in this country", added: "I am now able to give. I don't know that I've always done it in the best way. I have given a large amount to different causes and sometimes that has made me feel better. But the suffering of children so affected me that I wanted to set up my own charity.
"Now we are being recommended, referred and welcomed into care systems to help effect change. I was never arrogant enough to think I had all the answers. The solution was to get brilliant people on board."