The interpretation of the town's name Ystradgynlais has interested people over the years.
It is said to mean the 'Estate of Cynlais'. Cynlais, a ruler of this region, who lived in the first century, acquired the estate through marriage and was later made a saint. Another interpretation relates the name to 'the vale of the rough-sounding brook' from Nant Gyrlais- a stream which flows from the area of Penrhos to join the River Tawe just south of the town centre. Whichever interpretation is taken the town's 'modern history' is linked to the families of the Aubreys and Goughs. Sir Reginald Aubrey, one of Bernard de Neufmarche's Chief Knights, was rewarded with the lands of Abercynrig and Slwch including Palleg Estate and Ynyscedwyn. The Goughs married into the Aubrey family and developed the Ynyscedwyn Estate. This family arguably developed the town to what it is today.