The King sipped a ceremonial drink and was presented with a pig for feasting during a first day of events in Samoa.
The monarch was "dressed down", allowing a red garland to be placed around his neck when he was later honoured with a new title.
He was in good spirits despite the rain and clutched a bamboo-handled umbrella as he met locals.
The 75-year-old took part in a traditional 'ava ceremony, with the drink prepared by the daughter of Samoa's head of state.
As is custom, the King poured a few drops away before drinking deeply from a cup made of coconut shell.
He was wearing a short-sleeved white "Royal Navy"-style bush jacket and trousers he designed himself.
The jacket, similar to an elei, a smart casual short-sleeved shirt worn by Samoan men, was made by Anderson & Shepperd and featured fabric trim by the Samoan School of Arts.
The ceremony took place at the Pacific island's national university in a falesamoa, a large open-air covered hall made from timber.
A garland of dried fruit from the pandanus tree was placed around the King's neck at the end of the ritual, while the Queen was given a floral arrangement.
They were also given a selection of gifts, including a pig carcass for feasting.
The ceremony was watched by village elders and dignitaries including Samoan Prime Afioga Fiame Naomi Mata'afa and Foreign Secretary David Lammy.
Later, the chief of Moata'a village urged the monarch to "take a small part of Samoa away" with him as he made him Tui Taumeasina or King of Taumeasina.
"Once he has the title of Tui-Taumeasina we also become part of him and he will be connected to our people and our lands forever," said Tofaeono Lupati Fuatai.
The chief also praised the King for his efforts highlighting climate change, of which Samoa is particularly vulnerable to through events such as cyclones, flooding and storm surges.
"The King is the perfect advocate for us as a nation," he said.
"Climate change is real, it is now and we must urge the world to acknowledge how perilous it is for nations such as ours."
The royals' stay in Samoa follows their brief tour of Australia earlier this week when a senator controversially heckled the King in parliament.