The mystery benefactor has given £5.9million to St Michael le Belfrey, which is the parish church next to York Minster where Guy Fawkes was baptised.
The money will be used specifically to fund a major repair project to preserve the 16th-century building for future generations.
The Church of England also hope to turn St Michael be Belfrey into a 'stunning city centre venue that benefits York's residents and businesses.'
St Michael le Belfrey is a 'resource' church for the Diocese of York, meaning that it is used for the training and developing of new religious leaders. It also 'plants' new churches further afield, such as St Thomas's in Newcastle, and revives failing churches.
The renovations will include the stonework, stained glass windows and replacement of the roof. Alterations to the interior to make it more accommodating to modern worship will also be undertaken.
The donor approached vicar of 10 years Reverend Matthew Porter last summer to suggest the idea. A further generous donation was made previously in 2017 and a finance board has been appointed to manage the funds.
"I am one of only two people who know who the donor is. I can't say whether they're an individual, a family or an organisation, or whether they're from the UK or abroad," said Reverend Porter.
"The church is pre-Reformation and Grade I-listed. It's about 150 years since there was last any work done on the roof. We are going to do some serious renovation and make it great for the future.
"We are going to restore the stone to good condition, and our stained glass windows, along with York Minster's, are some of the best in the country. We are also going to install some new features - we're excited not just for ourselves but for the whole city."
The funding boost will allow the church to host conferences, concerts and events, hold outdoor baptisms and free up members of the congregation to spend more time on outreach projects working with young people and the homeless.
"The first Bishop of York, Paulinus, baptised the first northern king to convert to Christianity, Edwin of Northumbria, at the site of our church in the sixth century. We would love to introduce baptisms in the outdoor space we share with the Minster.
"We are so excited for this gift. It allows our committee members and congregation to spend more time on the bread and butter stuff. I'm not quite sure how we would have found the money without this donation - we would have had to turn to our Church family and applied for grants, but it is mostly covered now."