Sam Gould said the proportional representation element of the assembly electoral system would give UKIP "six or seven" seats, if it repeated its general election performance.
But he told BBC Wales the party was "definitely" capable of winning more.
UKIP got more than 200,000 votes in Wales in May, more than both Plaid Cymru and the Liberal Democrats.
It came second in six of the 40 Welsh seats but, under the first past the post system, did not win any.
The closest it came was Mr Gould in Caerphilly, who was beaten by Labour's Wayne David by around 10,000 votes.
UKIP's campaign in Wales also had several problems, including one of its candidates threatening to sue the party for libel, and its regional organiser stepping down.
The party believes the regional list system - whereby 20 of the 60 AMs are elected under a form of proportional representation - gives it a strong chance at the assembly election.
Former Conservative and UKIP MP Mark Reckless has been given the job of developing the manifesto and it is planning to appoint spokespeople on each of the main devolved policy areas.
Local branches will select the 40 constituency candidates by the autumn, while regional list candidates will be picked by a central committee.
Mr Gould said: "The (general election) performance was absolutely fantastic and we are so pleased with this foundation ready for next year's assembly election.
"If we were to run the same number of votes across Wales we would get seven AMs under the list system.
"So we're very confident of breaking through next year in the assembly election.
"I think seven seats would be a success, but I think we're in with a very good chance of achieving more."
He added: "Mark Reckless will be coming to Wales to work with a Wales-based policy team to help us turn the ideas we have from our local grassroots Welsh representatives into a reality in our manifesto."
There has never been a UKIP assembly member, but the party had a Welsh MEP elected in 2009 and 2014.
UKIP could win more than seven seats in the 2016 assembly election, the party's campaign manager for the poll has said.