The 2018–19 Welsh Football League Division One (known as the Nathaniel Cars Welsh Football League Division One for sponsorship reasons) was the 2018–19 season of the top football league in South Wales.
Together with its North Wales counterpart, the Cymru Alliance, the 16-team division formed the second tier of the Welsh football league system, one level below the nationwide Welsh Premier League.
Llanelli Town were champions in the previous season and were promoted to the 2018–19 Welsh Premier League.
Monmouth Town and Caerau (Ely) were relegated and replaced by the promoted teams from 2017-18 Welsh Football League Division Two.
They were Llantwit Major, Pontypridd Town and Ammanford.