Milan and Napoli are in mourning after Gennaro Gattuso's sister Francesca died aged 37.
Francesca, who had worked as a secretary at Milan, was taken ill in early February, with Gattuso absent from his usual post-match duties following the 4-2 win over Sampdoria having been notified of her condition.
Reports circulated early on Tuesday that Gattuso had been informed of her death during training, with the former Italy and Milan midfielder now managing Napoli.
Milan confirmed news of her death with a brief statement on Twitter.
It read: "Francesca Gattuso faced her illness with the same strength and energy she brought to Milanello [Milan's base] and Milan every day.
"Rino [Gennaro], the immense pain you and your family are feeling is also ours. Rest in peace, dear Francesca."
Napoli followed suit shortly after with a statement on behalf of president Aurelio De Laurentiis.
It said: "Aurelio De Laurentiis, with his wife Jacqueline and children Luigi, Edoardo and Valentina, the managers, the technical staff, the team, all of Napoli and Filmauro [Napoli's parent company], strongly embrace Rino Gattuso and join him in his immense pain for the loss of his sister, Francesca.
"We are all close to you with a dear heart, Rino."