Thales UK said the submarine was refitted with Thales's Sonar 2076 Stage 5 system and additional upgrades, such as Tomahawk cruise missile capability.
Thales UK was awarded a contract to upgrade three additional Trafalgar-class and three Astute-class submarines with the Stage 5 system by BAE Systems in February 2010.
The Stage 5 inboard replacement program is the latest in a series of developments to improve the capability, efficiency and through-life cost of the sonar system.
The upgrade also delivers an open architecture that allows a high degree of commonality with the Astute and Vanguard-class replacement submarines and supports the Ministry of Defense's vision for the evolution of a common sonar and combat system across the British navy submarine flotilla.
"We welcome the news that HMS Trenchant has re-entered operational service," said Phil Naybour, head of Thales UK's naval business. "Sonar 2076 Stage 5 represents a world-class sonar capability for the (British navy) and we look forward to working closely with our partners to upgrade the remaining Trafalgar- and Astute-class submarines."