I-63 was an Imperial Japanese Navy Kaidai-class cruiser submarine of the KD3B sub-class commissioned in 1928. She was sunk in an accidental collision with her sister ship I-60 with the loss of most of her crew in early 1939. The wreck was salvaged in 1940 and scrapped.

The Kaidai-type submarine was a type of first-class submarine operated by the Imperial Japanese Navy (IJN) before and during World War II. The type name was shortened to Kaigun-shiki Ōgata Sensuikan (海軍式大型潜水艦) Navy Large Type Submarine.