On 1 January 1904, Jessen was promoted to the rank of rear admiral. With the start of the Russo-Japanese War on 8 February, he was appointed deputy commander of Port Arthur and used the battleship as his flagship. However, on 10 March Vice Admiral Makarov reassigned Jessen to command the cruiser squadron based at Vladivostok, and he transferred his flag to the armored cruiser . This cruiser squadron conducted commerce raiding and other offensive operations while the remainder of the Russian Pacific Fleet remained bottled in Port Arthur by a Japanese blockade.