At least 10 women and five children were massacred in Hela Province, in a suspected revenge attack which reportedly left the victims mutilated and unrecognisable.
After news broke on Tuesday Mr Marape said it was one of the saddest days of his life and vowed to use "the strongest measures in law" against the killers.
On Facebook, the Prime Minister said the killings, which took place in his electorate, were led by gunmen from the Haguai, Okiru and Liwi tribes.
He said he had been requesting more police for Hela since 2012, adding that a local force of 60 officers could not support a province of 400,000.
"In memory of the innocent who continue to die at the hands of gun toting criminals, your time is up, before I had someone else to report to, now I have no one else to report to but the innocent you kill," he said.
"To Haguai , Okiru and Liwi, plus others in Tari who have been living off killing others, I am coming for you."
Photo PNG PM Media Caption: Papua New Guinea's Prime Minister, James Marape