This new deal means the brewers will remain shirt sponsors until September 2009 and is part of a wider four-year package under which they will have dispensing rights at the Millennium Stadium until 2012.
The Brains era has been a hugely successful one for Welsh rugby with Grand Slams being secured in 2005 and 2008, ending a 27-year drought.
The extended jersey sponsorship means Brains will now feature on the new Under Armour Wales kit which will be officially launched in October shortly before the autumn international series when New Zealand, South Africa and Australia all visit Cardiff.
WRU chairman David Pickering said: "The contract extension with Brains is a great boost for Welsh rugby.
"The partnership with Brains has been the envy of other rugby playing nations and the recent deal with Under Armour provides the WRU with a dynamic foundation of sponsors."