"Buchanan" departed San Diego for Southeast Asia on 27 January, 1968. During her deployment, she participated in Operation Sea Dragon (25 October 1966 – 31 October 1968) to disrupt the flow of supplies going from North Vietnam to support communist operations in South Vietnam. She also provided naval gunfire support (NGFS) to ground troops fighting in South Vietnam, most notably during the first phase of the Tet Offensive and the Battle of Huế (30 January – 3 March, 1968). During one tour providing NGFS, "Buchanan" came under fire from an enemy shore battery on 29 May. The ship sustained shrapnel damage to the heating and cooling system of the No. 1 cell of her ASROC launcher. Several of her antennae and electrical cables were damaged but no crewmembers were harmed. In response, "Buchanan" silenced the enemy position with 20 rounds from her 5-inch guns. In addition to her gunnery operations, "Buchanan" also served as plane guard in Gulf of Tonkin for (5–11 and 14–18 March), , and (15–17 May), in addition to and (20 June–1 July 1968). Buchanan retired to Subic Bay (9–17 June) for battle damage repairs after another tour providing NGFS to the I Corps Tactical Zone (30 May–7 June). During her time in the western Pacific, "Buchanan" was afforded a little rest and recreation at Hong Kong (7–11 May, 16–21 July) and Yokosuka, Japan (26–28 July) before setting a course home, stopping at Midway Atoll on 2 August and Pearl Harbor on 5 August along the way. Once back in San Diego on 11 August, 1968, her sailors were granted post-deployment leave for four weeks.