Jacob Thompson was diagnosed with Stage 4 high-risk neuroblastoma, a brain cancer, in 2014.
Last month, doctors at the Barbara Bush Children's Hospital in Portland in the US state of Maine told his parents that he only had a month or so to live.
Jacob then asked people to send him Christmas cards to help make his last holiday special.
Thousands of strangers answered the call with cards, some featuring his favorite animal -- penguins -- made it to his bedside at the Maine Medical Center, CNN reported.
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The child and his family had their early Christmas celebration on November 12 in the hospital, where he had been admitted since October 11.
On Sunday, Jacob died.
"It is with heavy hearts that we share the news of Jacob's passing with you. On Sunday, November 19th, 2017, at just 9 years old, Jacob passed away peacefully following his 4 year battle with neuroblastoma," his family wrote on Jacob's Facebook page.
"Each and every person who sent Jacob a Christmas card, a gift, a Facebook message or video, or a prayer made a difference in the final days of his life," they wrote.
"You brought Jacob joy, and you brought us all optimism for the future. Thank you for taking the time, and taking an interest in our sweet boy's journey.
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