"I-38" set out from Kure on 18 May 1944 with a Yokosuka E14Y1 (Allied reporting name "Glen") floatplane embarked to begin her third war patrol, during which she was to patrol in the Pacific Ocean east of the Marshall Islands and use the floatplane to conduct a reconnaissance fight over Kwajalein. After she put to sea, she received orders changing her reconnaissance target to Majuro. On 13 June 1944, the commander-in-chief of the Combined Fleet, Admiral Soemu Toyoda, activated Operation A-Go for the defense of the Mariana Islands and ordered the commander-in-chief of the 6th Fleet, Vice Admiral Takeo Takagi, to redeploy all 6th Fleet submarines to the Marianas. When the Marianas campaign began with the U.S. landings on Saipan on 15 June 1944, "I-38" and the submarines , , "I-38", "I-41", , and received orders to remain on patrol off the Marianas while the rest of the 6th Fleet′s submarines withdrew from the area.