The recent growth in the use of microfossil-based transfer functions in late-Quaternary sea-level reconstructions reflects their potential to accurately quantify palaeo sea-level changes. This study details the development of a diatom-based sea-level transfer function for the Severn Estuary, Southwest England, a macrotidal setting that experiences the second highest tidal range in the world. This setting presents difficulties in representing the full tidal range from mean sea level (MSL) to highest astronomical tide (HAT). However, two separate transects were merged successfully and a statistically significant relationship between contemporary diatom assemblages and altitude (m O.D.) was established. A diatom-based transfer function for palaeoaltitude was developed using weighted averaging (WA), tolerance downweighted weighted averaging (WA-Tol) and weighted averaging partial least squares (WA-PLS). WA-Tol produced the lowest prediction errors for altitude and the transfer function was applied to a fossil diatom data set from Gordano Valley, a site adjacent to the Severn Estuary.