Elidir Fawr is a mountain in Snowdonia, north Wales, the northernmost in the Glyderau. To the north of the summit is a small lake, Marchlyn Mawr, which is the upper reservoir for a pump-storage power station hidden inside the mountain itself. The lower reservoir is Llyn Peris. From the north, Elidir Fawr is very prominent, and can appear to be higher than the higher mountains behind it.
From Llanberis, the mountain is dominated by the former Dinorwic slate quarries and the waste they have left behind.
It is a reasonably short, but steep walk up to the summit, and this can be undertaken from the Deiniolen side or from Nant Peris. The Deiniolen walk provides views down to Llanberis, while the Nant Peris approach is short and quite steep.