Johnston has made history for the second time this year after becoming the first player to commit to the NRL's 19th club ahead of the Chiefs' entry in 2028.
The 31-year-old confirmed the move in an Instagram story post with the words "Yessssss sirrrrrrr".
But the expansion franchise's main target to be their inaugural marquee signing is still Jarome Luai.
The Wests Tigers superstar spent last weekend in PNG and met with Prime Minister James Marape and senior club officials.
"To be the first high-profile player to come and show interest in our team is something we deeply appreciate," Marape told local media.
"It sends a message that Papua New Guinea is ready, rugby league is strong here, and our future in the NRL is bright."
Johnston will join the new club on a one-year deal understood to be worth a tax-free $325,000.
He passed Ken Irvine's long-standing try-scoring record earlier this season with his 212th and 213th four-pointers for the Rabbitohs.
The 250-game veteran has since moved to a career tally of 219 tries.
Johnston, who will be 33 in 2028, received the blessing of Souths coach Wayne Bennett earlier this year to join the PNG franchise given his strong ties with the country.
The winger made his Test debut for the Kumuls in 2019, scoring four tries in 12 internationals for the league-obsessed nation.
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