Snow Patrol singer Gary Lightbody and Fall star Bronagh Waugh have also added their voices to the campaign.
Saturday's parade in Belfast city centre comes amid an ongoing political dispute on the region's ban on same-sex marriages.
The issue is one of the logjams in the ongoing negotiations to restore powersharing at Stormont, while the Democratic Unionist Party's (DUP) opposition to a law change has attracted increased scrutiny across the UK since the party became the Government's kingmaker at Westminster.
Amnesty International, which is one of the leading members of the Love Equality coalition organising the march, released statements from the celebrity backers.
"We've had enough of a history in our society in Northern Ireland of discrimination, mistrust and hatred," said Neeson.
"Yet, we're also an open-hearted, welcoming and terrific people. Let us show that to the world by treating gay, lesbian and transgender peoples as our brothers and sisters and allowing them to marry, if they so wish. By joining the rest of Ireland and other western countries in celebrating equality we can also attract more investment to our province.
"We have shown the world how we can affect a peace accord when many thought it impossible (1998 Good Friday Agreement).
"We must act on this and welcome all members of society through civil marriages, while still keeping our Christian values. Love, is love, is love."
Church leaders in Northern Ireland have consistently opposed changes to re-define marriage.
Speaking in response to the Scottish Episcopal Church's decision to allow gay marriages, a spokesman for the Presbyterian Church in Ireland told the Belfast Telegraph: "Many people in the Presbyterian Church in Ireland will be deeply saddened at this week's developments in Scotland, which seems so obviously at variance with the traditional biblical understanding of marriage as being between one man and one woman.
"The Presbyterian Church in Ireland affirms that clear position, which is based on the teaching of Scripture, and as a result, our ministers are not permitted to conduct, or to assist in leading, services of marriage for same-sex couples."
