46, was an Imperial Japanese Navy Kaichū-Type submarine of the Kaichū IV subclass.

The submarines of the Kaichu IV sub-class were an improved version of the preceding Kaichu III subclass, slightly larger, with heavier torpedoes, and with the deck gun mounted forward of the conning tower instead of aft of it.

For surface running, the submarines were powered by two 1,450-brake-horsepower (1,081 kW) Sulzer Mark II diesel engines, each driving one propeller shaft.

[2] This lasted until 1 December 1925, when the division returned to Submarine Squadron 2 in the 2nd Fleet.

[2] Ro-28 again returned to Submarine Division 14 on 1 December 1931, and remained in it for the remainder of her active career.

[2] In the years that followed, she served in the Kure Defense Division from 1 October 1932 to 1 February 1933 and from 1 December 1933 to 15 November 1934[2] and underwent a refit in 1934.

[2] She sank in an accidental flooding incident in the harbor at Wajima, Japan, in 1935, but was refloated, repaired, and returned to service.

[2] Ro-28 was decommissioned on 1 December 1938 and placed in Fourth Reserve in the Kure Naval District.