The group was having lunch in a pine forest in Castellana Grotte, near Bari, on Sunday afternoon when the area was hit by a sudden storm, Bari Today reported, and so they sought shelter in an empty nearby trullo, a white cone-roofed house.
Once inside, the lightning struck, with its dangerous current travelling through the trullo's wet floor.
The woman, who was taken to a hospital near Bari, was named as Emma Loliva, a teacher from Putignano.
A 51-year-old policeman was also seriously injured after suffering burns due to an electric shock.
The lightning caused electricity black-outs in other parts of Bari. Most of Italy was hit by strong storms over the Ferragosto holiday weekend.
Two young people also died in storms early last week.