The model and actress dated the American rapper on and off from 1994 to 2007 and they had three children together: twins Jessie James and D’Lila, 11, and son Christian, 20.
She also had a 27-year-old son, Quincy, from a previous marriage – it’s not known how she died.
Celebrities including Rihanna, 50 Cent and Missy Elliott have been paying tribute to her on social media.
Rihanna said: “We lost a real one today, beautiful inside and out. So many will be affected by this, some won’t even know why…but the whole world was graced by the vibrance in your spirit.”
Kim Porter posted a picture a few days ago on Instagram of Diddy, real name Sean Combs, with his family and the caption “Love”.
Just a few days ago, Kim posted a birthday message to Diddy, saying: “Thank you for giving me the best gifts of life – our children.”