The independent politician, who served as Ireland's President between 1997 and 2011, said the Church's stance had directly affected her son who is living in a same-sex relationship.
She was quoted by The Irish Times as saying: "My church's teaching on homosexuality is, in my view, evil.
"It conduces to homophobia; homophobia is evil.
"It ruins people's lives. It has ruined families' lives. It has caused people to commit suicide. It has caused people to live in dark shadows."
The 67-year-old spoke ahead of attending the pro-gay, lesbian, bisexual and transgender [LGBT] Dublin Pride event at the weekend - which she attended with her son Justin and his husband.
She said her son had been left in "the shadows of self-doubt [and] of misery of being really frightened" when he learned the church "has a view of him that is inimical to the way God made him".
McAleese also described treatment of the LGBT community by the Catholic Church as "unchristian" and "worse than uncharitable".
During a public interview given at the Missionary Society of St Columban in County Meath, she also criticised the Pope for not being progressive enough regarding the role of women in the Church.
Dublin's annual Pride parade drew tens of thousands of people to the streets, including Ireland's first openly gay prime minister, Leo Varadkar.
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