Sales went up 9% from August and 4% year-on-year, according to the Vietnam Steel Association.
Industry leader Hoa Phat Group posted its biggest sales last month since the beginning of the year.
The company saw construction steel sales rising 15% from August to 352,000 tons.
Vnsteel saw sales rising 8% from August to 265,000 tons.
The increase in construction steel demand came thanks to major infrastructure projects being developed such as North-South highways and airport upgrades.
Steel prices have been steady throughout September and at the lowest level since 2020, which stimulated demand.
But overall, the steel industry is still seeing low demand, the VSA said. In the first nine months 7.7 million tons of construction steel were sold, down 20% year-on-year. Exports fell 13% in the same period.
Prices are set to remain at current levels until the end of the year, according to a KB Vietnam Securities forecast.
A hike in domestic demand is expected early next year as property projects see difficulties in acquiring permits will be resolved, it added.