Cardiff was officially recognised as the capital of Wales, UK on December 20. 1955.

Senedd – National Assembly for Wales, Cardiff
Part of the United Kingdom’s Landmark Buildings set 
Issued in 2017
Designed by GBH
The Welsh Parliament (Senedd) opened in 2006. 
The Senedd: A Landmark Building
The architects behind this building have a fascinating page on the building Senedd Cymru, Welsh Parliament – Civic – Projects – RSHP

Prior to the 1950s, Wales did not have a recognised capital city. Polls conducted by newspapers in the preceding decades showed a desire for the country to have a capital, but councils were split on where it should be, with Caernarfon, Aberystwyth and Swansea all making compelling claims to be a candidate for the role.

However, after much debate, Cardiff eventually won out, being officially announced as the capital of Wales in 1955. Following the announcement, the city became home to a number of national institutions, including the Arts Council of Wales, Sport Wales National Centre, and, later on, the Senedd, headquarters of the Welsh Government.
Introducing Cardiff | Wales.com

 

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