Williams confirmed the sad news on Twitter, posting an image of him holding his grandmother's hand tightly.
He wrote: "Last night I lost one of my most loved, my nana. Such a beautiful soul may the most high have mercy on her Duas pls.
Yesterday saw families across New Zealand celebrate Mothers Day, and Williams has tweeted since that we should cherish every day we have with our mothers and grandmothers.
We should cherish/visit our mothers/grandmothers as much as possible because you never know when it's there time #EverydayIsMothersDay," he added.
Williams was evidently close to his nana, who was in his profile picture on Twitter, and he has often posted about their close relationship.
His sister Niall tweeted an image last August that he quoted, writing: "nana and pop" with loving emoji faces.
Williams has been training with New Zealand's sevens squad ahead of the Paris and London legs of the sevens world series.
And he's still hoping to make the cut for Gordon Tietjens' squad for the Olympics in Rio in August.