Gaining nominations from 18 of the Church's 19 presbyteries, the 57-year-old will succeed Revd Ian McNie at its General Assembly in June.
A father-of-three and originally from Coleraine in County Londonderry, Revd Sellar will occupy the most senior office within the Presbyterian Church in Ireland.
He is currently minister of East Belfast's Bloomfield Presbyterian Church and he is the son of late Robert G Sellar, an artist who also spent 20 years ministering in the Republic of Ireland.
Commenting on his appointment to the office of moderator, Revd Sellar said: "It has been a huge privilege to have been involved in a lifetime of leadership in local congregations, south and north.
"I'm humbled that the wider church has trusted me with this responsibility, granting fresh opportunities of mission, which this year as Moderator of the General Assembly will bring, not only serving the wider church, but society throughout the whole of Ireland and further afield."
Revd Sellar was selected following a three-way election involving Revd Tony Davidson of the First Presbyterian Church in Armagh and Revd Robert Bell of Ballyclare Presbyterian Church in County Antrim.
Revd Sellar, who will become the 177th Moderator of the Presbyterian Church in Ireland, says he enjoys walking his labradoodle, current affairs, broadcastings and skiing occasionally.