Mynydd Twyn-glas is a sprawling 472-metre-high mountain extending into Caerphilly and Torfaen county boroughs in South Wales.

It lies southwest of Pontypool, west of Cwmbran and east of Newbridge and taking the form of a rolling plateau the margins of which are defined by steep slopes, many of which are cloaked with conifer plantations.

The thickly afforested northern slopes of the hill drop down steeply into Cwm y Glyn.

The NE-SW aligned Glyn Fault cuts through the northwestern corner of the hill.

In addition a number of public footpaths cross the hill and provide access from neighbouring towns and villages.