The second Ro-55 was an Imperial Japanese Navy Kaichà « type submarine of the K6 sub-class. Completed and commissioned in September 1944, she served in World War II and was sunk during her first war patrol in February 1945.
The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth. They displaced surfaced and submerged. The submarines were long, had a beam of and a draft of . They had a diving depth of .
For surface running, the boats were powered by two diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a electric motor. They could reach on the surface and underwater. On the surface, the K6s had a range of at ; submerged, they had a range of at .
The boats were armed with four internal bow torpedo tubes and carried a total of ten torpedoes. They were also armed with a single L/40 anti-aircraft gun and two single AA guns.
Ro-55 was laid down as Submarine No. 396 on 5 August 1943 by Mitsui Zosensho at Tamano, Japan. She was launched on 23 April 1944 and was renamed Ro-55 that day, the second Japanese submarine of that name. She was completed and commissioned on 30 September 1944.
Upon commissioning, Ro-55 was attached to the Maizuru Naval District and assigned to Submarine Squadron 11 for workups. She was reassigned to Submarine Division 34 in the 6th Fleet on 4 January 1945.
On 27 January 1945, Ro-55 departed Kure, Japan, to begin her first war patrol, assigned a patrol area in the South China Sea west of Mindoro in the Philippine Islands. While in the Philippine Sea east of Luzon on 2 February 1945, she reported that Allied aircraft had attacked her and that she would reach her patrol area five days late. The Japanese never heard from her again.
After dark on 7 February 1945, the United States Navy destroyer escort detected a surfaced submarine on radar while escorting a Leyte Gulf-bound Allied convoy off Iba, Luzon. As she closed the range, the submarine submerged, and at 23:30 Thomason began attacks against it, firing 24-projectile Hedgehog barrages that sank the submarine at .
The submarine Thomason sank probably was Ro-55. On 1 March 1945, the Imperial Japanese Navy declared her to be presumed lost off the Philippine Islands with all 80 men on board. The Japanese struck her from the Navy list on 10 May 1945.