Built by VSEL at Barrow-in-Furness, Triumph was commissioned in October 1991 and is the seventh and last of the Trafalgar class.
"HMS Triumph sails for sea trials with significant safety and capability improvements, equipping her to provide a substantial contribution to Defence during her second commission," Captain Mike Robinson, the Superintendent Submarines for Devonport, said.
The work, the last Trafalgar class submarine refit and refuel to be undertaken, was carried out in partnership with the Royal Navy's major contractor, Babcock.
The refit has taken six years to complete and cost nearly £300 million.
Over 1,000 personnel have worked on the submarine in this time, totalling 2.75 million man-hours, including Babcock employees, ship's staff and various other external contractors.
Capability improvements have included installation of the latest sonar systems, and an upgrade for the Tomahawk land attack cruise missile system.
A new command and control system has been installed, as well as a new internal fibre-optic computer systems network and enhanced satellite communications system.
An additional ballast pump has been installed to aid rapid deballasting, and a number of safety improvements added to fire-fighting and escape capabilities.
HMS Triumph will now undergo a three-month sea trials program before returning to the Royal Navy's operational fleet for ongoing duties.
And you thought that Rankin and Sheean being out of the water for four to five years was bad.