The launch of the future HMS Artful took place on Friday and Saturday at the company facility in Barrow-in-Furness, Cumbria, with the vessel being moved from its construction site to a dock site.
"The launch of Artful is another significant step forward in the Astute program, said BAE Systems Astute Program Director Stuart Godden.
"Building on past experiences, we've been able to launch her in the most advanced state of construction of any submarine to be built in Barrow. This allows us to now fully concentrate on the test and commissioning activities required to get her to sea."
Astute attack submarines are 318 feet long, 37 feet in the beam and have a submerged speed of as much as 30 knots.
Complex testing of the Artful's systems will now take place, followed by sea trials next year.
BAE Systems said remaining Astute-class submarines on order for the Royal Navy are in various stages of construction.