The non-swimmer from Ivory Coast was attempting to rescue her three-year-old son from the lake near Crans Montana on July 6th when she herself fell in, 20 Minuten reported.
The woman was in turn rescued and flown by an air rescue helicopter to hospital in Sion where she died of her injuries on Tuesday.
The toddler was unharmed.
Valais cantonal police said it was not clear how the child had managed to fall in to the lake.
There are frequent reports of accidents or drownings in Swiss lakes and rivers in the summer months, often caused by inexperienced swimmers overestimating their own ability.
Every year warnings and advice are issued to those seeking to cool off in Swiss waters.
But even experienced swimmers are not immune to risk. Earlier this month a man died while taking part in the annual Lake Zurich Seeüberquerung in which thousands of swimmers take part.