Lanracader [= Lanraeader] 1254, Rauraeadur [= Lanraeadur] c.1291, llanrraydr’ 1377, Llannrayndir Mochnant 1406, Lanreadir, -dre c.1495, Thlanrayerd 1533-8, lan hrayadyr y mochnant c 1540, llann rrayadr c.1553, Llandrayade Ymoughnant 1559, LL.Raiadyr ymochnant c. 1562, LL.rhayadr 16g., Llanrhaidrymochnant 1836

Welsh llan and river-name Rhaeadr with the definate article and name of its historic cantref (hundred) Mochnant. Rhaeadr (Reader flood 1592, raider flu. 1578) itself is W rhaeadr, 'waterfall, rapids' presumably taken ultimately from the name of its well-known waterfall Pistyll Rhaeadr, 'waterfall spout', (pistill, 'spout, well'). Identical to Llanrhaeadr-yng-Nghinmeirch Denbighshire which is Lanrayadyr 1254, Llanraadour in Kymergh 1390 and (o) lann rhayadr ynghemmeirch c.1600-20