"I am so happy for our neighbours to the south," Ontario's Kathleen Wynne posted on Twitter after the landmark decision.
The U.S. case was spearheaded by 83-year-old Edith Windsor, a New Yorker, who married partner Thea Spyer six years ago in Canada, where same-sex marriage has been legal for a decade. The couple's marriage was recognized by New York state, but not by the U.S. federal government.
In a second Tweet, Wynne called the ruling "an important step" for the United States, and included a link to a video in which the Ontario premier talks about the day she married her partner, Jane Rounthwaite.
"My own partner Jane was very emotional on that day because she didn't believe in her lifetime she would ever see such a thing," the 60-year-old premier said in the video.
"It was the culmination of a long struggle, and in fact we got married because the rules had changed and because my own children said because we could, we should."
Once people get to know each other, once there's a relationship and a trust built-up, then issues of sexual orientation don't matter, added Wynne.
"I've been travelling to every corner of the province since I became premier and not once has the issue of my sexual orientation, my lifestyle come up," she said.
"It doesn't surprise me, but it is gratifying to feel that my belief in the people of Ontario is absolutely well founded, and that we're on the road to being an even more inclusive province than we've been in the past."
Wynne, who is Ontario's first female leader, will also become the first sitting premier to take part in Toronto's annual gay pride parade on Sunday, which brings thousands of visitors to the city.
"Pride festivities across Ontario are vibrant celebrations of the lives and stories of our LGBTQ communities," she said in a statement.
"As the battle for equality rages on, even among our closest neighbours, we will showcase our progressive values on the world stage."
The U.S. high court also cleared the way Wednesday for the resumption of same-sex marriages in California, the most populous U.S. state.
U.S. President Barack Obama praised the court's ruling on the federal marriage act which he said "was discrimination enshrined in law."
"It treated loving, committed gay couples and lesbians as a separate and lesser class of people," Obama said in a statement. "The Supreme Court has righted that wrong and our country is better off for it."
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