HMS Prince of Wales has been floated and moved to her fitting-out berth at Rosyth.
In a delicate overnight operation, sluices were opened and water gradually filled the dry dock, specially enlarged for the 65,000-tonne warship and her older sister HMS Queen Elizabeth.
She was then moved from her home for the past three years to the neighbouring basin and J and K berths, where her sister was fitted out before departing on sea trials during the summer.
Minister for Defence Procurement Harriett Baldwin said: "This is an important moment in the monumental programme to build these two magnificent ships. I would like to thank the 10,000 people from across the UK who have helped us make such progress during 2017 on both HMS Queen Elizabeth and HMS Prince of Wales."
Crew and engineers from the Aircraft Carrier Alliance monitored the process throughout, while a flotilla of tugs moved the leviathan the short distance to her new berth.
"For me, seeing water surround the ship has really made it transform from being a ship in build to a ship preparing to go to sea," said Sub Lieutenant Freddie Spreckley, who's just joined the ship as a marine engineer after completing his professional training at HMS Sultan in Gosport.
"I was privileged to be one of the last few people to walk underneath the ship before the sluices opened and flooded the dock. It was very exciting - a once-in-a-lifetime opportunity - to be involved with this engineering milestone in the ship's life and historic moment for the Royal Navy."
Captain Ian Groom, the carrier's Senior Naval Officer, said that the effort made by teams from industry, the Ministry of Defence and the ship's company to reach this milestone had been "immense".
He continued: "It has been a massive team effort and I am proud of every individual contribution. It is a fitting end to the Year of the Navy to have HMS Queen Elizabeth commissioned and HMS Prince of Wales undocked and afloat for the first time.
"As the second of her class, HMS Prince of Wales is of strategic significance ensuring continuous carrier strike capability. Working as one team we are delivering an unmistakable sign of commitment to the defence of our great nation and that of our allies."
Prince of Wales is 3,000 tonnes heavier than her sister was at the same stage - as the second ship in the class, construction and fitting out has moved more swiftly thanks to the lessons learned building HMS Queen Elizabeth.
Originally planned for 2018, the 'undocking' of the carrier took place ahead of schedule and just three months after the carrier was officially named by HRH the Duchess of Rothesay, as the Duchess of Cornwall is titled in Scotland.
The next milestone in the ship's life will be the first running up of the generators and gas turbines, bringing the ship to life, which are earmarked for middle of 2018, followed by sea trials in 2019.
