Mynydd-bâch (Welsh for 'little mountain'; more commonly spelled today as Mynyddbach, Mynydd-bach or Mynydd Bach) is a hamlet in Monmouthshire, Wales.

The northwestern part of Mynydd-bâch is underlain by Brownstones formation sandstone formed between 419.2 and 393.3 million years ago (mya) in the Devonian period.

This is surrounded by a margin of between 40 and 100 metres width of inter-bedded Devonian quartz and sandstone conglomerate formed between 372.2 and 358.9 mya, running roughly under most of Shirenewton Primary School and along the southern part of the road, continuing under the Carpenter's Arms public house before turning north.

[5] It includes Whitemill Common and Roughets Wood which are separated by Mounton Brook; they had been planted with spruce, larch and poplar from the mid-20th century.

As part of continuing environmental improvement plans, many conifers were removed from 1995 and replaced with broadleaved species.

The riverside environment is now native again and semi-natural, containing species associated with ancient woodlands, including wood-sorrel, bluebells, primroses and pendulous sedge.

[6] There are two local Sites of Special Scientific Interest:[7] the Llywyn-y-celin wetland bordering Mounton Brook, to the west of Roughets Wood, has a spring mire and swamp plant environment that is rare in southeast Wales;[8] Mwyngloddfa Mynydd-bâch is a hibernaculum for lesser horseshoe bats in an old mine adit in the sandstone hillside.

The prize-fighter, Bill Benjamin, was its most famous landlord; he lost to the English champion, Tom Sayers, in three rounds in 1858.

[25] A ghost tale was reported of a young girl murdered by her father and buried in the pub walls.

[30] For Monmouthshire County Council, Mynydd-bâch is in the electoral ward of Shirenewton, and the Devauden area committee.

[7][29] The member of parliament for the area is Catherine Fookes of the Labour Party, representing the Monmouthshire constituency as of 2024.