Jozef Wesolowski, who was defrocked after rising to the role of Archbishop, was due to be tried by the Vatican over the accusations.
His case was due to be heard in July, but Polish media reported that was put back after he suffered a heart attack.
News of his death arose on social media with confirmation from the Vatican following soon after. A spokesman said initial investigations suggested the death was due to natural causes. He also said Pope Francis has been fully informed.
Writing on Twitter, Catholic Priest Piotr Studnicki said "Today at 5 am died Archbishop #Wesołowski. God is righteous and merciful and took the matter into his own hands. Lord grant unto him eternal rest."
Dzis o 5 rano zmarl abp #Wesolowski. Bóg sprawiedliwy i milosierny wzial sprawe w swoje rece. Wieczny odpoczynek racz Mu dac, Panie...
— Piotr Studnicki (@piotrstudnicki) August 28, 2015Wesolowski served in the Dominican Republic as the senior Vatican official in the country.
He was recalled in 2013 after rumours of alleged abuse surfaced. He was then defrocked in 2014 and placed under house arrest.
It's reported the 67-year-old was in Rome awaiting trial.
The Pope has spoken often over the last two years of the need for the Church to toughen up on abuse.