The minister also promised to update the law on parenting of children by gay couples.
Mr Shatter's backing for same-sex marriage is significant as he will be in charge of drafting any laws in the area.
Mr Kenny keeps kicking to touch on the question of gay marriage, saying he cannot express a preference as he speaks for the Government. But Mr Shatter joins Tanaiste Eamon Gilmore in clearly saying he favours gay marriage.
Although he stressed it was a personal view, Mr Shatter notably made his remarks in a speech to the Gay and Lesbian Equality Network.
"I believe a democratic republic that professes a commitment to equal citizenship should not continue to prevent same-sex couples from entering into a legal partnership . . . designated as being a marriage," he said.
The minister said he was "conscious not every one shares my view" on the issue.
The new Convention on the Constitution will consider the question of gay marriage in April.