The prime minister made the announcement in a public address last night after a national emergency meeting in the capital Port Moresby to discuss the country's Covid-19 response strategy.
Mr Marape's declaration comes after health authorities in the country confirmed the first case of the Covid-19 coronavirus on Friday.
"We have now declared a nationwide state of emergency. I request with this announcement here that our nation gives greater support to this call," he said.
"This call is not to make anyone panic as we speak there is only one pronounced candidate [with Covid-19] and by the grace of God there is no indicating evidence of the expansion of this virus as yet."
Mr Marape said as part of the state of emergency all domestic flights in the country would be grounded from tomorrow and all public transportation on the country's roads are asked to cease operations.
All schools in the country are also being closed.
"Starting Tuesday 24th of March this country will come to a lockdown for a period of 14 days," he said.
"This step is absolutely necessary for us to make sure that every citizen remains where they are. I do not intend to raise hype and tension and panic. It is just about you staying at home so we can take stock," he said.
Mr Marape said more meetings would be held today to further define the national Covid-19 response strategy before more announcements are made about expectations for public behaviour during the lockdown.
Solomon Islands announced border closure
The Solomon Islands government yesterday closed the country's border to non-citizens as a preventative measure against the Covid-19 coronavirus pandemic.
The country is yet to register a case of Covid-19. Samples from three suspected cases sent to Australia last week all came back negative for the coronavirus.
Prime Minister Manasseh Sogavare said the border closure was a necessary precaution to try and prevent the entry of the coronavirus and also to prepare health authorities to respond to an outbreak if it did get through.
The government was implementing price control measures for basic goods and essential services and would take all necessary measures and actions to first prevent and - if it comes to it - control the infection and spread of Covid-19 while also maintaining economic and social stability.
Mr Sogavare called on all Solomon Islanders to heed public health warnings and to remain united in the fight against the global coronavirus pandemic.