On 25 October 2009 Lydiate was named in the Wales national squad for the November 2009 international series, making his debut as a second-half replacement against Argentina on 21 November 2009. Following the withdrawal of captain Ryan Jones from the Wales squad to face Australia in the final Autumn International, Lydiate made his first test match start at flanker on 28 November 2009. He was named in the Wales squad for the 2010 Six Nations Championship, however did not play a single game. He then broke into the Welsh first XV in the autumn internationals of that year, starting against Australia, New Zealand and Fiji as well as being named in the team to face South Africa before withdrawing with a calf injury. He won the Man of the Match award against the Fijians, where he wore the Number 7 jersey for the first time.

He played every match of Wales’ 2011 Six Nations Championship campaign as well as all four warm-up matches. In August 2011 he was named in the Wales squad for the 2011 Rugby World Cup in New Zealand. He played in Wales’ opening two games against South Africa and Samoa, but missed the pool games against Namibia and Fiji with an ankle injury that lead to him infamously waking up to ice his leg every hour. It paid off, and he played a key role in Wales’ quarter-final win over Ireland and then subsequent losses to France and Australia. He finished second on the tournament’s top-tacklers, behind only Wales and Dragons teammate Toby Faletau.

In the 2012 Six Nations Championship, Lydiate received the Player of the Tournament award after featuring in every game bar the opener against Ireland and receiving man of the match awards against Scotland and France.

In April 2013 Lydiate was named in the squad for the 2013 British and Irish Lions tour to Australia.

On 30 October 2014 it was announced Lydiate would be leaving Racing Metro and returning to Wales on a dual contract between the Welsh Rugby Union and a Welsh region, on 7 December 2014 this was announced to be Ospreys