Ro-31, originally named Submarine No. 70, was an Imperial Japanese Navy Kaichu-Type submarine of the Kaichu V (Toku Chu) subclass. After a diving accident in 1923 prior to completion, she was salvaged, rebuilt, and completed in 1927. She served in a training role during World War II, surrendered at the end of the war in September 1945, and was scuttled in April 1946.