Chadwick Boseman kept his cancer diagnosis private while "bringing joy and courage" to children with the disease.
Unbeknown to fans, the movie star discovered he had colon cancer in 2016 and filmed some much-loved roles "during and between countless surgeries and chemotherapy".
Boseman went to see young patients at a US hospital treating childhood cancers and paediatric diseases, taking them toys.
In one picture, posted on Twitter in 2018, he helped a girl celebrate her birthday.
St Jude Children's Research Hospital paid tribute to "our friend Chadwick Boseman", saying it was "deeply saddened" to hear of his death, aged 43.
A statement on Twitter added: "Two years ago, Chadwick visited the St Jude campus and brought with him not only toys for our patients but also joy, courage and inspiration.
"Our thoughts are with his family and friends at this time."
That year, Boseman became emotional discussing the impact of Black Panther on two boys who had terminal cancer and had wanted to see the film before they died.
"There are two little kids, who recently passed from cancer, and throughout our filming, I was communicating with them, knowing that they were both terminal," he said in the Sirius FM interview.
"What they said to me, and their parents said, is 'They're trying to hold on, until this movie comes'.
"To a certain degree, you hear them say that and you're like, 'I've got to get up and go to work, I've got to learn these lines, I've got to work on this accent...'
"To a certain degree it's a humbling experience because you're like, 'This can't mean that much to them'.
"But seeing how the world has taken us on, seeing how the movement has taken on a life of its own, I realised that they anticipated something great.
"I think back now to (being) a kid and just waiting for Christmas to come, waiting for my birthday to come, waiting for a toy that I was going to get a chance to experience, or a video game.
"I did live a life waiting for those moments and so it put me back in the mind of being a kid, just to experience those two little boys' anticipation of this movie."
The Hollywood star broke down in the interview, recorded before the film's release.
"When I found out that they (died)" he said, in tears and unable to finish, "it means a lot."