USNS Relentless (T-AGOS-18) was a Stalwart class Modified Tactical Auxiliary General Ocean Surveillance Ship of the United States Navy. Stalwart class ships were originally designed to collect underwater acoustical data in support of Cold war anti-submarine warfare operations in the 1980s. Ex-USNS Relentless is now National Oceanic and Atmospheric Administration research vessel NOAAS Gordon Gunter (R 336).