AgNb7O18 is a relaxor ferroelectric with a Burns temperature of similar to 490 K and an incipient transition to the nonergodic state. The short-range structure is shown by convergent-beam electron diffraction to have the polar space group Im2m, but refinements against powder X-ray diffraction find the long-range structure to have the centrosymmetric space group Immm. Relaxor behavior in AgNb7O18 appears to originate from the partial occupation of large interstices by Ag+ cations. Both cations and oxygen anions are displaced away from zones where NbO6 octahedra are edge-sharing.