The 36-year-old can be seen running towards the ocean at Guaruja resort in Sao Paulo.
Her arms are outstretched as she warns her little boy of the storm overhead.
In the next image lightning strikes the woman, killing her instantly.
Her body is obscured by a Mitsubishi truck, but others on the beach are horrified.
Husband Leandro Lopes Santos told the Daily Mail: “The weather started to close in and we decided to leave. I was putting our bags into the car, and the kids were on the beach.”
“The next minute my wife was lying on the floor. I received a mild shock.”
The Baviati family had gone to the beach at midday but three hours later they saw the weather become overcast.
Arionaldo Garrido said: “Several rays had already flashed across the sky and streaked down.
“Suddenly we heard a loud bang and saw several people running. It's a real tragedy for the family who have lost a wife and mother.”
Lightning is common in the area at this time of year, according to climatologists.