The service in the shadow of Tower Bridge on the River Thames, London, marked Severn's official return but the ship has been fully operational since July last year following a refit.
The Offshore Patrol Vessel was re-commissioned last summer, becoming the first Royal Navy vessel to have been brought back to life since the Falklands conflict in 1982.
The primary role of HMS Severn in her second life is a combination of navigational training, protection of UK waters and fishery protection.
She was decommissioned in October 2017 after 14 years' service, chiefly patrolling UK fishing grounds to ensure trawlers were sticking to regulations.
Watch: HMS Severn returns to the Navy fleet with a colourful new look.
However, 12 months later she was deemed too important to UK defence to be disposed of and it was announced she would return to the fleet.
The vessel also now has a new lick of paint with a Second World War-era 'Western Approaches' scheme.
Commander Philip Harper, Commanding Officer HMS Severn, said: "Bringing Severn back from the dead has been an amazing experience and commissioning here alongside Belfast, with both of us in our World War II camouflage, is the culmination of 18 months of hard work and dedication."
The re-commissioning service in the capital on Saturday was attended by friends, families, affiliates and senior naval officers.
With a crew of about 45 sailors, the ship regularly rotates one-third of her crew, allowing her to be available for operations up to 320 days of the year.