Plaid Cymru is celebrating an emphatic victory in the by-election to choose a new assembly member for Anglesey.
Rhun ap Iorwerth, who retained the Ynys Mon seat for Plaid, said it could be a "catalyst and springboard" for the party in Wales.
The ex-journalist said there would be no "miracles" but issues such as jobs and salary levels would be tackled.
He secured a 9,166 majority over Labour, who were pushed hard by UKIP in third place.
The Conservatives' vote collapsed while the Lib Dems were beaten into last place by Socialist Labour.
The by-election had been prompted when former Plaid Cymru leader Ieuan Wyn Jones stood down in June.
Former BBC Wales presenter Mr ap Iorwerth, 40, was cheered by Plaid supporters on the steps of the Senedd on Friday afternoon when he arrived in Cardiff Bay with his family to be sworn in as an AM.
"Lessons have been learnt here and I believe it (the victory) can be a catalyst, and also a springboard, an energising force for the party both locally and nationally," he said.
"That can be seen in the enthusiasm of our supporters on the island and further afield - and that enthusiasm is not going to go away."
Speaking on BBC Radio Wales he added: "I want to build a society on the island, and in the wider community, that is fairer and prosperous. We will do things bit by bit."
In the poll Labour managed to secure second place despite the efforts of UKIP who recorded their best ever result in an assembly seat with third place.
The Conservatives' vote collapsed by 20% while the Lib Dems were beaten into last place by Socialist Labour.
Labour's Tal Michael said he was disappointed but there seemed to be a pattern of Plaid doing well at assembly elections on the island, and Labour better in General Elections.
"It's not what we hoped for; we had hoped to break that mould."
Mr Michael said the party fought a positive campaign and the two parties who had done that had done the best at the polls.
UKIP's Nathan Gill hailed his "fantastic" result of third place, just over 300 votes behind Labour and said people were looking at the bigger picture on jobs and immigration.
"People are tired of the 'same old, same old,'" he said.
"Keep an eye out for us. We know in 2016, we will get AMs elected. People are as Euro-sceptic here as in the rest of Britain."