Presenting research conducted by the ‘St Stephen’s Chapel, Westminster’ project at the University of York, this article focuses on the Great Seal devised in 1649 and re-issued in 1651 to enable the Commonwealth to function following the execution of Charles i. As a familiar and ancient image of monarchy, the Great Seal posed an obvious challenge to the authority of the Rump Parliament. A radical new design, authorised by parliamentary committee and executed by engraver Thomas Simon, replaced royal iconography with images of popular sovereignty and nationhood: a map of England and Ireland on the obverse of the Seal, and the interior of the House of Commons chamber (formerly St Stephen’s Chapel) on the reverse. The result was a striking evocation of political authority located in the House of Commons and deriving from the English people. Engravings of the Commons chamber, in circulation since the 1620s, are identified as a probable source for Simon’s work. The Great Seal also re-asserted England’s dominion over Ireland and the waters surrounding the British Isles. Overall, this article argues for continuity as well as alteration in the iconography of the Great Seal of England, at a time of revolutionary political change.