Emergency services were called after reports that two men were seen jumping into the Menai Strait between the north Wales mainland and Anglesey.
Two RNLI lifeboats and the North Wales Police helicopter attended after the pair were seen making the 150ft (45m) leap.
It is thought the two men then swam to the Anglesey side of the strait where they were arrested.
Boat users in the Menai Strait made a may-day call around 1215 BST.
A Beaumaris RNLI spokesman said: "It's rare to have two people going off the bridge at the same time."
The Britannia is one of two bridges between Anglesey and the mainland.