The dynamic duo have both inked two-year deals with the Cowboys from next season after playing key roles in the Sharks' regular finals appearances during their tenure for most of the past decade.
The acquisitions, however, suggest off-contract Queensland representative winger Murray Taulagi could be playing the last of his eight seasons at the club.
The 27-year-old Samoa and Australia representative was offered an extension by the club but did not take it up.
Taulagi has been linked with a move to Sydney Roosters, who will lose star winger Mark Nawaqanitawase back to rugby union next year.
The Cowboys have struck an impressive balance this year between local products and valuable imports in their rise into the top eight.
Forward recruits Reed Mahoney (ex-Canterbury), prop Thomas Mikaele (ex-Warrington) and Matt Lodge (ex-Manly) have all hit their straps.
Katoa and Ramien will join a backline that includes speedy local North Queensland products Tom Chester and Jaxon Purdue.
Purdue, currently playing centre, is set to move to fullback next year when No.1 Scott Drinkwater departs for St George Illawarra.
That will allow powerhouse Ramien to partner Chester in the centres.
Winger Katoa, 28, knows how to find the tryline as his 85 tries in 134 NRL matches for the Sharks and four tries in 10 Tests for Tonga demonstrates.
The flamboyant flyer goes by the moniker of "The Tongan Fisherman" on Instagram and will enjoy his move north where he can pursue his noted passion for angling.
Ramien, 29, is highly regarded for his toughness and dynamic running and has played 176 NRL games for the Sharks and Newcastle, while scoring 58 tries.
Cowboys recruitment manager Clint Zammit said the pair would add experience and class.
"The signings of Jesse and Sione significantly upgrades the level of depth in our backline," Zammit said.
"They join Tom Chester, Braidon Burns, Rob Derby, Jaxon Purdue and James Walsh in being signed until at least the end of 2028, which creates great stability in the backline for our club.
"Jesse has been in the top echelon of centres in the NRL for the better part of a decade. He's experienced, can break tackles and is a strong defender.
"Sione is a wonderful finisher and carries the ball strongly out of his own end."
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