Last Known Activity Grayback’s tenth patrol, her most successful in terms of tonnage sunk, was also to be her last. She sailed from Pearl Harbor on 28 January 1944, for the East China Sea. On 24 February Grayback radioed that she had sunk two cargo ships 19 February and had damaged two others (Taikei Maru and Toshin Maru sunk). On 25 February she transmitted her second and final report. That morning she had sunk tanker Nanho Maru and severely damaged Asama Maru. With only two torpedoes remaining, she was ordered home from patrol. Due to reach Midway on 7 March, Grayback did not arrive. On 30 March ComSubPac listed her as missing and presumed lost with all hands.
From captured Japanese records the submarine's last few days can be pieced together. Heading home through the East China Sea after attacking convoy Hi-40 on 24 February, Grayback used her last two torpedoes to sink the freighter Ceylon Maru on 27 February. That same day, a Japanese carrier-based plane spotted a submarine on the surface in the East China Sea and attacked. According to Japanese reports the submarine "exploded and sank immediately," but antisubmarine craft were called in to depth-charge the area, clearly marked by a trail of air bubbles, until at last a heavy oil slick swelled to the surface. Grayback had ended her last patrol, one which cost the enemy some 21,594 tons of shipping.
Grayback ranked 20th among all submarines in total tonnage sunk with 63,835 tons and 24th in number of ships sunk with 14. The submarine and crew had received two Navy Unit Commendations for their seventh, eighth, ninth, and tenth war patrols.
Grayback received eight battle stars for World War II service.