A video posted online by the site Mediaite shows Biden addressing supporters Saturday at a party sponsored by the State Society of Iowa.
"You saw what America came to see. You saw early on, Iowa Democrats saw early on the incredible promise and the incredible capability, the incredible leadership of the guy I now serve with," Biden began during his address.
"I'm proud to be president of the United States, but I'm prouder to be Barack ..." he said as laughter erupted in the room, CNN reported.
"I am proud to be vice president of the United States," he corrected himself, adding "Well, there goes that."
This isn't the first time Biden has been referred to as the president of the United States. President Barack Obama made the same gaffe in August 2008 when he introduced Biden as "the next president of the United States."