The historic ‘Yes’ vote will see the denomination become the first Anglican Church within the UK to allow gay couples to marry.
The majority of the Anglican communion still oppose same-sex marriage.
Ahead of the vote, the conservative Anglican group Global Anglican Future Conference said: “If this action is taken it will further marginalise faithful Anglicans in Scotland who seek to uphold Jesus’ teaching on marriage.”
The Church’s governing body, the General Synod, will debate a motion to change canon law on marriage, in Edinburgh today.
The Synod ruled last year that the issue of same-sex marriage should be discussed across all seven dioceses. Six of the seven voted in favour of changing the law, with the diocese of Aberdeen and Orkney opposing the move.
The motion needs support from two-thirds of the Synod’s three houses - Bishops, Clergy and Laity.
A ‘conscience clause’ included in the motion would grant churches an ‘opt-out’.
Primus of the Scottish Episcopal Church, the Most Rev David Chillingworth said the church would ‘seek to reach a decision on the question while sustaining its unity in Christ.’
He added: “At this General Synod we reach the final stage of our discussion of marriage and of same-sex marriage in particular.
“The Scottish Episcopal Church expresses a diversity of views on this question. Those views are held with integrity.
“To reach a decision while sustaining unity in Christ will require both humility and generosity on all sides.”
Same-sex marriages have been legal in Scotland since the end of 2014.