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.
