Inoke Siolongo Filivaolelei Tonga was arrested and charged with Poli Kefu's death in May.
The 27-year-old pleaded guilty at the Nuku'alofa Supreme Court on Thursday.
He will be sentenced on October 6th.
Matangi Tonga reported Poli Kefu died after being hit with a rock.
It said Poli Kefu, who hailed from Lapaha, was not only a respected Tonga Red Cross communications officer, but was president of the Tonga Leitis Association.
He was also recognised for his work as a LGBTQIA+ rights advocate in Tonga and the region.
