The carved chair, known as the Bishop's Seat and used for dignitaries, was stolen from All Saints Church in Ashwicken, near King’s Lynn, sometime between 19 August and 23 August.
Norfolk Police said it was thought to be worth between £500 and £800.
Churchwarden Steve Williamson said: "The theft of the chair is a great disappointment and robs us of an important part of our heritage. As such, the value of it is far less than it may fetch for the thieves."
Mr Williamson added: "The church, however, remains open, despite the risks, as it provides a safe and spiritual place for all at any time and especially for those who may be distressed."
PC Jonathan Phillips said: "We are keen to hear from anyone who may have information.
"This item is of both historical significance and also an asset for the local community."