The American company replaced Reebok as the kit supplier for all Wales international teams in 2008.
Whereas the original Under Armour contract was worth roughly £2.5m a year, the new deal will see the annual fee rise to around £3.7m.
This renewal will be the first major deal overseen by new WRU chief executive Martyn Phillips.
His predecessor, Roger Lewis, stepped down after Wales' exit from the 2015 World Cup having held the role since 2006.
It was announced in September that Wales' home ground, the Millennium Stadium, will be renamed the Principality Stadium in a 10-year sponsorship deal beginning in January 2016.
Under Armour's other high-profile clients include Tottenham Hotspur, American football player Tom Brady and golfer Jordan Spieth.

Cuadrilla's plans for fracking at its Preston New Road site at Little Plumpton were approved in October.
The firm says it will put Lancashire first for creating jobs, new skills, investment, and community initiatives.
Opponents said they were surprised by the news since a legal challenge to the government's decision is outstanding.
Preston New Road Action Group issued formal legal proceedings at the High Court in November.
Cuadrilla said an "independently audited tracker" will monitor its pledges to Lancashire.
Chief executive Francis Egan said the shale gas industry would be good for the county's economy.
"We expect that in the exploration phase alone we will spend in the order of Â£50m - clearly all of that will not go to Lancashire... but we would hope that up to half of that money could get spent in Lancashire," he said.
The firm also said Bolton-based civil engineering firm AE Yates Ltd has been awarded the site's Â£1.5m building contract.
Work is expected to start early next year, Cuadrilla said.

First Minister Alex Salmond confirmed the project will receive an award of £2.2m.
The government has a target for the equivalent of 100% of Scotland's electricity consumption to come from renewable sources by 2020.
Environmental charity WWF Scotland welcomed the news saying it would bring extra jobs as well as cheaper energy.
The funding will go to The Carbon Trust's Offshore Wind Accelerator (OWA) programme, a joint industry and government scheme, which brings together nine offshore wind developers with over 72% (31GW) of the UK's licensed capacity.
The OWA will receive £200,000 in 2014/15 and £2m in 2015/16.
Funding will be used to encourage wind developers to work together to try and meet cost reduction targets and also to commercialise offshore wind turbines for Scottish waters.
The first minister said that renewable energy was "extremely valuable" to the Scottish economy and investment in renewable energy would make sure the country could comfortably meet its energy demands.
He added: "Scotland is admired around the world for our work in renewable energy and in 2013 we set a new record for renewables generation, emphasising our commitment.
"The Scottish government's target is to generate the equivalent of 100% of Scotland's gross annual electricity consumption from renewable sources by 2020, as part of a wider, balanced, energy mix, and we are already nearly half way there.
"Renewable energy is extremely valuable to Scotland's economy, to reducing our carbon emissions and in providing low carbon energy supplies as well as jobs and long term investment."
The Carbon Trust's Chief Executive Tom Delay added: "We are delighted to be working with the Scottish government to drive further cost reductions in the offshore wind industry.
"The sector has huge potential to generate low carbon power and create economic value. Key to that success will be driving costs down through innovation and doing this quickly.
"The OWA and this new injection of funding will be key to help meet this cost reduction challenge."
News of the funding was also welcomed by environmental protection charity WWF Scotland. The group's climate and energy policy officer Gina Hanrahan said: "It's great to see the Scottish Government supporting projects to help lower the costs of offshore wind.
"Studies estimate that Scotland's offshore wind industry could create 28,000 jobs by 2020 and contribute over £7bn of investment to the economy.
She concluded: "By driving innovation and promoting collaboration, initiatives like the Accelerator Programme will help to ensure that we make the most of the economic and environmental benefits of offshore wind."
The Welsh Rugby Union is set to agree a new nine-year contract with kit manufacturer Under Armour worth £33m.
[NEXT_CONCEPT]
The company behind the UK's first horizontal fracking operation has announced six "commitments" which it claims will ensure Lancashire benefits.
[NEXT_CONCEPT]
The Scottish government has announced funding for a initiative to reduce the cost of offshore wind energy.