Ifor Bach (meaning Ivor the Short) (fl. 1158) also known as Ifor ap Meurig and in anglicised form Ivor Bach, lord of Senghenydd, was a twelfth-century resident in and a leader of the Welsh in south Wales.
At this period the Normans had conquered England, but large areas of Wales were still under the control of the native Welsh Princes and Lords. Whilst parts of the old Welsh Kingdom of Morgannwg (which was to become Glamorgan) had fallen to the Normans, Ifor ap Meurig held land in Senghenydd, a region of Morgannwg which had not yet fallen completely. Broadly Senghennydd was the upland area bounded by Brecknock to the north, between the River Taff and the Rhymney River and abutting Cefn Onn in the south.
The Norman Lord of the region was William Fitz Robert, 2nd Earl of Gloucester.
In 1158 he attacked and killed Morgan ab Owain, Welsh Lord of Caerleon and Gwynllwg (anglicised as Wentloog).
According to Giraldus Cambrensis in the Itinerarium Cambriae, in 1158 Ifor Bach was a tenant of William, Earl of Gloucester, who held Cardiff Castle. Gloucester was trying to take land which under Welsh law belonged to Ifor.
Ifor scaled the walls of Cardiff Castle using his bare hands, seized the Earl, his Countess Hawise, a daughter of the Earl of Leicester, and their young son Robert, and kidnapped all of them to the woods of Senghenydd. He refused to release them until he had recovered the land he had lost "and a lot more".
He married Nest, sister of Rhys ap Gruffudd (also known as The Lord Rhys).