Falcons swoop on two US Eagles
Back row forward Tameilau, 26, has been capped four times by the United States national team, the Eagles, this year and is renowned for his physical ball-carrying and hard-hitting defence.
Civetta, also 26, brings line-out ability, physicality and athleticism, with the ability to play at both lock and back row.
Newcastle Falcons Director of Rugby Dean Richards said: "Both David and Nick fit the profile of what we are looking for in our forward pack for next season.
"David is a destructive runner who offers us something different in attack, and has the ability to put on some big defensive hits. He has masses of ability and I believe could go on to become a world-class player.
"Nick is a rounded forward who will add to our physical threat, and we look forward to working with both of them when they arrive for pre-season."
Tameilau helped the USA to second place in the inaugural Americas Rugby Championship, starting at No 8 against Canada, Chile, Brazil and Uruguay.
The forward was a talented American football (Gridiron) player before specialising in Rugby Union, excelling at defensive end for San Francisco City College and West Texas A&M following the completion of his Mormon mission.
Brought up in a rugby-playing family, Tameilau represented the US at Under-18, Under-19 and Under-20 levels and has most recently been playing for San Francisco in the inaugural PRO Rugby competition.
Civetta joins from Italian Serie A club RC I Medicei, who are based in Florence, having previously played for fellow Italian sides SS Lazio and Viadana.
The New Yorker took up rugby at the age of 18 when he attended the University of Notre Dame, and has played three times for the US Select (America's second team) after twice being named in the USA Rugby Collegiate All-American team.
The American pair take Newcastle's off-season player influx to nine, following the previously-announced arrivals of Vereniki Goneva (Leicester), Sam Egerton (England Sevens), Joel Hodgson (Yorkshire Carnegie), Sam Lockwood (Jersey), Harrison Orr (Ealing), Evan Olmstead (London Scottish) and Ben Sowrey (Worcester).