"If two people of the same sex fall in love and want to marry, why would our government stand in their way?" Casey said in a statement that included "support for marriage equality."
"At a time when many Americans lament a lack of commitment in our society between married men and women, why would we want less commitment and fewer strong marriages?" he said.
Casey had previously supported civil unions for same-sex couples but opposed allowing marriage, making that position clear as recently as last year when he ran for re-election.
But he had become an outlier within his party, which has prominently embraced gay rights. Last week, as the Supreme Court heard two same-sex marriage cases, a slew of Democrats changed their positions to favor allowing gay and lesbian couples to wed.
Pennsylvania gay-rights advocates peppered Casey's office with phone calls as he remained quiet about his views.
"If the president of the United States can come out fully in support for marriage equality and win reelection, there's really very little fear that other people should have," said Ted Martin, executive director if Equality Pennsylvania, which led the phone effort.
In a statement, Martin said Casey's new position "puts him squarely on the right side of history."
In contrast, Michael Geer, president of Pennsylvania Family Institute, said Casey yielded to pressure from the gay rights lobby and compared him unfavorably to his father, Gov. Casey.
Geer said the senator's shift, which also included a call to repeal the federal Defense of Marriage Act, "is so much different than the character and fortitude of his father, Gov. Casey, who famously and courageously bucked his party's stand" for increased access to abortion.
"It's very troubling to see Sen. Casey respond to special interest pressure on an issue as important as this one," Geer said.
Public polls nationally and in Pennsylvania show attitudes moving dramatically in favor of same-sex marriage. In Pennsylvania, 52 percent of voters favor allowing same-sex marriage, a 19 point jump from 2006, according to a Franklin & Marshall poll completed Feb. 3.
The poll found that 36 percent of Pennsylvania voters "strongly favored" allowing same-sex marriage, up 19 points from 2006, while those "strongly opposed" dropped 16 points, to 34 percent.
Terry Madonna, a Franklin and Marshall College political scientist, said Casey has never relied on social issues as central to his appeal.
"Social issues don't define Casey," he said.
Casey won a second six-year term in November. Since then he has also reversed his long-held position on new gun laws, coming out in favor of bans on assault weapons and high capacity magazines and for tougher background checks in response to the Newtown school shooting.
Casey's stand on same-sex marriage puts him in line with other local Democrats, including U.S. Reps. Bob Brady and Allyson Schwartz.
Local Republicans largely oppose same-sex marriage. Sen. Pat Toomey (R., Pa.) has said marriage should be between a man and a woman and a spokeswoman said Monday his position hasn't changed.
U.S. Rep. Jim Gerlach (R., Pa.) "has been consistent in his support of the existing federal law that defines marriage as the union of one man and one woman" but backs civil unions, a spokesman e-mailed.
Contact Jonathan Tamari at email@example.com or follow on Twitter @JonathanTamari. Read his blog 'CapitolInq' at www.philly.com/CapitolInq.