The Samoa Rugby Union has confirmed the appointment at a press conference in Apia.
Tietjens will be aided by assistant coach Stephen Betham, with Ramsay Tomokino named as team manager.
Tietjens met with the Samoa Rugby Union board last night, including Chairman and Prime Minister Tuilaepa Sailele Malielegaoi, to finalise details of his contract.
The 60-year-old spent 22 years in charge of the New Zealand sevens team, stepping down from the role in September after their fifth place finish at the Rio Olympics.
