As Company Secretary he will be responsible for ensuring the rules of corporate governance within the WRU are complied with and standards of financial and legal practice are maintained.
He will attend WRU Board meetings and the Group's Annual General Meeting to offer advice and guidance to directors and representatives of all the affiliated organisations.
Mr Williams will continue in his senior role at Hugh James and carry out his Company Secretary duties in an honorary capacity.
Hugh James has a long standing professional relationship with the WRU and has recently offered legal advice to the Union on several important and high profile matters.
Gareth Williams has maintained a close, professional relationship with the WRU Group for almost two decades as lead solicitor to the union and has acquired expert knowledge about the organisation, its operational structure and systems of corporate governance.
He became Senior Partner at Hugh James in 2005 and was previously head of the company's Business Litigation Division. He is regarded as an expert in sports law and substantial commercial litigation.
A Welsh speaker he is also a trustee of the Welsh Rugby Charitable Trust and a Governor of the University of Glamorgan.
The Chairman of the Welsh Rugby Union, David Pickering, said: "We are delighted that Gareth has agreed to take on the role of Company Secretary to the WRU.
"The union will benefit from his expertise, knowledge and mature, intelligent counsel which I am certain will serve us well as an organisation going forward.
"The member clubs and affiliated organisations must be assured that the constitution of the WRU is honoured and protected at all times and Gareth is the ideal candidate to fulfil the role of guardian of that corporate governance."
The Group Chief Executive of the WRU, Roger Lewis, said: "During my time at the WRU Gareth has proved to be a trusted professional colleague and good friend of the organisation.
"He is a highly respected lawyer and his counsel is always based on sound legal knowledge combined with a finely tuned intellect and common sense.
"He is always calm and reasoned in his advice and is very experienced at operating alongside the most senior executives and directors in very high profile organisations."
Gareth Williams said: "I am delighted to have been asked to become the Company Secretary of the Welsh Rugby Union.
"As well as my strong professional ties with the WRU I have a close affinity with rugby so I am looking forward to being able to serve the union in this new role.
"I have an excellent legal and administrative team working with me at Hugh James so I am confident I can free the time to fulfil the WRU role with the energy and focus it deserves."
He replaces the late Mike Jefferies, a close personal friend, who died suddenly last year after joining the WRU as Head of Legal Affairs in 2007 following a successful career with Hugh James.
Gareth Williams takes up the unpaid role of Company Secretary with immediate effect.