Hancock said the legislation will change the preamble to the Marriage Act that currently defines marriage as an institution between a man and a woman to make it gender neutral.
He noted the change is symbolic. The federal government made same-sex marriages legal in 2005.
"The Marriage Act is one that is outdated and there have been concerns raised about it," Hancock said.
"Some people find it offensive that (the reference) is still in the preamble to the Marriage Act, so it's appropriate for us to update that."
The preamble, passed in 2000, currently reads that "marriage between a man and a woman has from time immemorial been firmly grounded in our legal tradition, one that is itself a reflection of long-standing philosophical and religious traditions."
The decision was made in the Progressive Conservative caucus earlier Thursday. Hancock dismissed suggestions it was a divisive debate.
"It was very good today in caucus in terms of understanding how and why we should move forward on this," he said.
There will also be changes to allow transgender persons to change their birth certificates without having to first undergo gender reassignment surgery.
Hancock said the change to the Vital Statistics Act will remove barriers for transgender persons to get identification as needed.
"The reality is (that) gender is no longer the identifying characteristic that is used in documents like drivers licences," he said.
"There are better identification characteristics and technology.
"It's not important at all as an identifier, and it matters to people (to change it)."
Hancock said the changes will come in before the spring session ends around the middle of May.
Opposition parties and advocates lauded the decision.
"It's a message to Canada that the old stereotypes of Alberta as being the redneck, roughneck oil executive province have changed," said Kris Wells, with the University of Alberta's Institute for Sexual Minority Studies and Services.
The decision comes days after some Progressive Conservatives and the opposition Wildrose party were criticized for joining forces to defeat a non-binding motion to urge schools to bring in gay-straight alliances.
Such alliances provide peer support to help gay students avoid bullying.
The motion, presented last week by Liberal Kent Hehr, was supported by the Liberals and the NDP.
Some PCs supported the motion, and Hancock has said if he had been there for the vote, he would have supported it as well.
Those who voted against it said that while they were for gay-straight alliances, it was best to let schools come up with their own ways to combat bullying.
That led to criticism the two right-of-centre parties are quick to talk about equality, but reticent to act on it.
Earlier Thursday, about 25 people joined politicians from all parties, including Hancock, on the steps of the legislature to show support for gay-straight alliances.
NDP Leader Brian Mason reminded the crowd it was the Tories and the Wildrose who engineered the motion's defeat.
"I don't know if you notice that whirring sound," said Mason to the crowd, referring to the distant hum of machinery.
"That is the sound of two conservative parties backpedalling as fast as they possibly can."
Hancock said the decision to amend the Marriage Act was not tied in any way to criticism from the failed motion.
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