Mr Key will lead a Parliamentary delegation to mark the occasion.
The Prime Minister has invited the Leader of the Opposition David Shearer and representatives of each political party in Parliament to travel to Samoa on the visit.
"I look forward to meeting Prime Minister Tuilaepa and members of his Cabinet while in Samoa to mark the 50th anniversary, as well as discussing regional issues of mutual interest ahead of this year's Pacific Islands Forum," he said.
Mr Key will have a state lunch at the Robert Louis Stevenson Museum where the Treaty of Friendship was signed.
He will launch Friendship Week which will include a New Zealand film and television festival, a food and beverage mini-expo and a series of cultural events and exhibitions.
Samoa has a unique historical relationship with New Zealand, having been administered by New Zealand from 1914 until 1962.
Samoa gained independence from New Zealand in 1962 and signed the Friendship Treaty.
"Samoa is the only country in the world with which New Zealand has a formal Friendship Treaty - it's a mark of our deep ties, shared values and long history of cooperation," he Mr Key.
In the last census 131, 103 Samoan people were recorded to be living in New Zealand.
High levels of Samoan migration to New Zealand started in the 1950s and by the late 1970s Samoan illegal immigrants were targeted in the notorious dawn raids.
Other delegates:
Hon Hekia Parata, Minister of Pacific Island Affairs
David Shearer, Leader of the Opposition
Hon Phil Goff, Labour foreign affairs spokesman
Rt Hon Winston Peters, NZ First leader
Metiria Turei. Green Party
Te Ururoa Flavell, Maori Party
Hon Peter Dunne, United Future leader
Hone Harawira MP, Mana Party leader
Peseta Sam Lotu-Iiga, National
Su'a William Sio, Labour
Kris Faafoi, Labour
Asenati Lole-Taylor, NZ First