Pencader Tunnel is an abandoned railway tunnel on the old trackbed of the Carmarthen to Aberystwyth main line in south-west Wales. It was originally bored for the Llanpumsaint-Pencader section of the Carmarthen & Cardigan (C&C) Railway.

It is approximately 985 yards (901 m) long (just over half-a-mile), making it the longest tunnel in West Wales and is bored through Ordovicianshales. It is single-track and built to the C&C's adopted broad gauge dimensions, as opposed to the smaller standard gauge used on Britain's railways today. It is also known as alternatively Alltwalis Tunnel and Dolgran Tunnel to local villagers.