Herefordshire, Gloucestershire is to the east. The Bristol Channel forms the south eastern boundary. To the south west is Glamorganshire and to the north west Brecknockshire.
For additional information see Wikipedia: Historic Monmouthshire and GENUKI:Monmouthshire
Please note we have followed the GEN UKI convention of showing Monmouthshire as a Welsh county. On FamilySearch you will find it listed as an English county.