The Tamar class lifeboat - John D Spicer - began its journey to north Wales from the RNLI's headquarters in Poole, Dorset, on Friday.
A new boathouse will be built with materials brought in by sea, due to problems with the cliff-side location.
Volunteer crew members have been training on the new boat, which is faster than the current one.
As it sailed in, sirens, foghorns and flares were let off in welcome as more than 100 people cheered.
The old lifeboat entered service in 1987 and has been launched 315 times.
Source: RNLI
"When the new lifeboat comes around the headland for the very first time I know there will be gasps from everyone who has turned out to welcome her," said Porthdinllaen RNLI coxswain Mike Davies speaking before the boat arrived.
"Bringing her home will definitely be one of the highlights of my career and a proud moment.
"This really is a fantastic boat and I have every confidence that she will serve us well and help us to save more lives off Porthdinllaen," he added.
The new craft was funded by a bequest from John Dominic Spicer, from Oxfordshire, who died in October 2010.
At the request of the executors of Mr Spicer's will, the new boat is called the John D Spicer.
The new boat will be temporarily kept on a mooring while work to build a new boathouse at the Morfa Nefyn site begins.
It had been hoped that construction on the new building would begin last spring, but that has now been put back.
There had been concerns locally that materials for the new building would have to come through Nefyn and along a cliff-top road by the local golf club.
Source: RNLI
Last year the charity said it had taken the decision to look at transporting materials for the new boathouse by sea because of the financial implications.
"The close co-operation and assistance we have received from the Nefyn and District Golf Club and the local community has been fantastic, however it just was not a financially viable option to re-align the trackway away from the cliff edge," Colin Williams, divisional inspector of lifeboats in Wales, said at the time.
It is not the first time the RNLI has used the sea to help construct a new boathouse.
This method was used to build the £5.5m boathouse at Tenby, Pembrokeshire, in 2005.
More than 100 people have welcomed a new £2.7m lifeboat for Porthdinllaen on the Llyn peninsula.