They indicate hunter-gatherers used the River Wye for food and transport some 6,500 to 7,500 years ago.
The late Mesolithic items show there were settlers in the area thousands of years earlier than previously thought.
The artefacts were found – during gas mains work – under a former riverbank where the River Wye used to flow before it changed course.
Elizabeth Walker, curator of Palaeolithic and Mesolithic Archaeology at National Museum Wales, examined the items after being alerted by Monmouth archaeologist Steve Clarke.
Ms Walker said: “It’s a nice little group of later Mesolithic flints – middle Stone Age – they are probably around 6,500 or 7,500 years old.
“Among the items are two little flint barbs which would have been hafted [attached] onto a piece of wood or antler and used for fishing or hunting.
“We have also got a scraper from there which might have been used for cleaning the skins or scraping bark and twigs.
“There were also quite a few waste pieces of flint used in making the tools.”