In a 5-4 ruling, the court ruled that the 1996 Defense of Marriage Act is unconstitutional because it denies married same-sex couples the same benefits that heterosexual couples receive.
The court ruled that DOMA singled out same-sex marriage and treated same-sex couples "as living in marriages less respected than others."
Married same-sex couples in a dozen states and the District of Columbia will now be eligible for more than 1,000 spousal benefits previously off limits under DOMA. And the court's other decision on Wednesday, striking down California's Prop 8, paves the way for same-sex marriages to resume in that state.
Income tax: Same-sex couples will now be able to file their federal income taxes jointly. For many couples -- especially those where one person earns significantly more than the other -- merging incomes for tax purposes will result in big savings.
Janet and Janet Emery-Black, who married in California in 2008, estimate they could save more than $10,000 a year in income tax by filing jointly. Because one is retired and the other works full-time, combining incomes would put them in a much lower tax bracket.
The Emery-Blacks are also hoping to take advantage of a protective refund claim by amending their taxes for the past three tax years to get back some of the extra income tax they paid as a result of DOMA.
The couple has already amended their 2009 tax returns and they plan to file protective refund claims for 2010 and 2011. If the IRS approves those claims they expect to receive a total of more than $30,000 in refunds for the last three years.
Other couples won't be helped by filing jointly, however. Kevin O'Leary and Brian Esser, from New York City, will actually end up owing roughly $5,000 more in income tax per year because they bring in similar salaries.
Gift tax: Same-sex couples will also be exempt from gift tax when transferring assets to each other. Under DOMA, any gift between same-sex spouses of more than $14,000 began adding up to a lifetime limit of $5.25 million -- after which a 40% tax was assessed. Opposite-sex couples have never been subject to that tax.
Joshua Hatfield Charles and Dixon Charles, from Rockville, Md., were married five years ago. They paid more than $8,000 for attorneys and accountants to help avoid a huge gift tax for transferring assets to each other. If DOMA hadn't been overturned, they estimate they would have spent another $7,000 to $9,000 on legal fees and taxes.
Same-sex couples who divorce can also be subject to the federal gift tax when dividing assets.
Health insurance: Some couples also pay extra income tax on the medical benefits one partner receives through the other's health insurance plan.
The Emery-Blacks, for example, will save at least $1,000 in income tax per year by not paying tax on these spousal benefits.
Meanwhile, many federal employees will likely be granted spousal benefits like partner health insurance.
For Joanne Pedersen, a retired federal employee from Connecticut, the ruling could mean that her retirement plan will finally extend health benefits to her wife, Ann Meitzen. It would be a big boost: Meitzen suffers from lung illnesses and spends half of her monthly Social Security benefits on medical expenses.
"Financially we're not doing great," said Pedersen, now 59. "We were looking forward to a nice retirement where we would both be doing well, but that's not happening."
Death benefits: Same-sex couples will now be eligible for the same federal tax treatment and Social Security benefits as opposite-sex couples in the event that one spouse passes away.
This means a surviving spouse will be eligible for Social Security survivors benefits and will be exempt from the 40% federal estate tax on assets exceeding $5.25 million.
What the decision doesn't do: While many of the specifics of the Supreme Court's ruling are still fuzzy, it appears that couples must be married at a state level for all of these federal benefits to apply -- meaning domestic partnerships and civil unions don't qualify, Young said.
It's also unclear whether federal benefits will apply to same-sex couples who marry in a state where same-sex marriage is legal but move to a state where it's not recognized.
Currently, same-sex marriage is legal in 12 states and Washington, D.C., while seven states grant domestic partnerships and civil unions.
"[The Supreme Court is saying,] 'Yes, you can have these benefits, but up it's up to each state whether they are even going to recognize these couples in the first place,'" said Kyle Young, a financial advisor and vice president for Wells Fargo Advisors. "It's still going to leave us in the position where there's [still] a patchwork of marriage equality."