Which ship was closest to the Titanic but did not respond to its distress calls?

The SS Californian was the closest known ship to Titanic that failed to respond promptly to her distress signals.

Californian was a British Leyland Line cargo-passenger ship stopped in an ice field north of Titanic on the night of 14–15 April 1912. Her wireless operator, Cyril Evans, had warned Titanic about ice earlier, but he later shut down the wireless set and went to bed. Titanic’s distress messages therefore went unheard aboard Californian.

Officers on Californian saw rockets rising from a nearby vessel and tried signalling with a Morse lamp, but they did not treat the rockets as a clear distress call. Captain Stanley Lord was informed, yet the ship did not begin steaming toward the apparent emergency until after dawn, when news of Titanic’s loss arrived by radio.

The ship’s exact distance and the identity of a “mystery ship” seen by some Titanic witnesses remain disputed. Nevertheless, Californian is the standard answer because official inquiries identified her as the nearby vessel whose crew observed signals but did not render timely assistance. RMS Carpathia was the ship that responded and rescued survivors.