Some £85,000 was spent by the council to improve a nine mile (14.5km) section of the A4059, between Aberdare and Abercynon, which is prone to flooding.
It was part of a £23m scheme to improve roads in the area.
Council leader Andrew Morgan said road improvements "continue to be a key priority when it comes to allocating our financial resources".
The road, known locally as the "Letty Turner's bends" has also had its drainage system improved.
Councillor Morgan said: "We will be investing over £23m in highways improvements, infrastructure and strategic projects over the next three years - this is on top of the £53m we have already invested since 2011 meaning that by 2018/19 we will have invested over £76m in these key areas in recent years."
Work to help prevent flooding on a main road in Rhondda Cynon Taff has been completed.