A US politician has admitted to taking a glass of water that touched the lips of Pope Francis and swilling the leftover liquid with family and colleagues.

Congressman Robert "Bob" Brady watched as the Pope addressed the US Congress, the pontiff occasionally sipping from a glass of water on the lectern.

Congressman Bob Brady drinks from a glass of water that Pope Francis used during his speech to Congress. Credit:AP

When the speech was over, half a glass of liquid was left behind. That's when Brady struck.

Casually taking the glass, with one finger on the bottom and another on the rim, he walked back to his office, where he took a sip, gave one to his wife, and encouraged his staff to do the same, the Philadelphia Daily News reported.