Oksana Grigorieva said Monday the concert will be for affluent Russians and the proceeds will pay for vital surgery for children born in the area affected by the radioactivity.
Grigorieva told journalists she had already funded operations for 50 children with heart conditions and leukemia in Ukraine, Belarus and Russia.
Grigorieva and Gibson split earlier this month after a three-year romance. She said they had agreed to raise their baby daughter Lucia together.
The radiation left after the meltdown at the Chernobyl plant still affects thousands of children born in the area.