Vice President Joe Biden joined in greeting the senator.
The moderate Specter announced Tuesday he was switching parties , citing, among other things, philosophical differences with GOP conservatives and a recognition that he likely would lose the Pennsylvania Republican primary in 2010.
Obama said he didn't expect Specter to be a "rubber stamp" for the administration's position on legislation or issues.
"I know the decision Senator Specter made yesterday wasn't easy. It required long and careful consideration, and it required courage," Obama said. "But I know that it also reflects an independence that has been the hallmark of Arlen Specter's career since the days he arrived in Washington. He has never been in the Senate to fight for any particular party, but rather for the men and women of Pennsylvania who sent him here."
Biden said he considered Specter to be a friend, confidant and partner on major legislation and issues.
"(It) gives me great pleasure, great pleasure, Mr. President, to now officially be in the same caucus with Arlen Specter," Biden said.
"I'm even more pleased that Arlen's independence, integrity and piercing intellect will now be sitting in a Democratic caucus. I think it will be a real added value."
Specter said he has represented the people, not the Republican Party, in Pennsylvania.
He told Obama and Biden, "You have projected an administration that I feel very comfortable with."