The Battle of Laoshan (Chinese: 老山战役), known in Vietnam as the Battle of Vị Xuyên (Vietnamese: Mặt trận Vị Xuyên) was fought in 1984 between China and Vietnam as part of Sino-Vietnamese conflicts (1979–1991) during the Third Indochina War. It is considered the largest scale engagement involving both countries since the 1979 Sino-Vietnamese War.

Prior to the battle, Vietnam was still dealing with the Cambodian–Vietnamese War and FULRO insurgency. Starting from 1983, Vietnamese border raids in Thailand increased as Vietnam aimed to eliminate the remnants of the Khmer Rouge forces who fled there. The remnants still engaged in guerrilla warfare aiming to attack Vietnamese occupation forces in Cambodia. China had pledged support to the resistance fighters and in February 1983, Yang Dezhi of the People's Liberation Army General (PLA) Staff Department visited Thailand promising to provide aid should they ever be attacked by Vietnam. In April 1983, Li Xiannian (elected President in June) warned Vietnam that China would not stop its support for Thailand as long as Vietnam kept up its aggression towards it. In December 1983, Deng Xiaoping met Norodom Sihanouk of the Coalition Government of Democratic Kampuchea. Sihanouk requested help from Deng as Vietnam forces were continuing to attack resistance group positions.

As a result of its previous pledges, China planned to attack Laoshan to relieve Vietnamese pressure on resistance forces. Commander-in-Chief of the Royal Thai Army, Arthit Kamlang-ek was invited by China to visit the battlefield and to discuss with PLA officers how they fought against the People's Army of Vietnam (PAVN).

Laoshan is a range of hills located on the China–Vietnam border between Malipo County, Yunnan and Vị Xuyên, Hà Giang. It consists of three ridges, radiating from its main peak which is 1,422 meters high. It is unclear why China chose to attack at Laoshan as it was mountainous and was difficult to reach from anywhere except Hà Giang. However, there were several beliefs. One view was that since it was also challenging to reach from Hanoi, attacking it could divert many PAVN troops away from other regions, or even capture the provincial capital itself – a major psychological blow. It is also believed it was selected because it would show to Thailand that China would not tolerate any loss of territory not matter how remote it was and also force Vietnam to break its concentration on Cambodia and Thailand.

There were two components to the offensive. Project 14 was a ground assault and Project 17 was an artillery bombardment. From 2 to 27 April 1984, China had conducted the heaviest artillery barrage since 1979 against the Vietnamese border region, with more than 60,000 shells pounding 16 districts in Lạng Sơn, Cao Bằng, Hà Tuyên, and Hoàng Liên Sơn Provinces. 256 artillery pieces bombarded the areas with 414 targets being allegedly hit. Decoy ground attacks were used in an attempt to lure PAVN forces into the open so the artillery could hit them. This was accompanied by a wave of infantry battalion-sized attacks on 6 April. The largest of them took place in Tràng Định District, Lạng Sơn Province, with several Chinese battalions assaulting Hills 820 and 636 near the routes taken during the 1979 invasion at the Friendship Gate. Despite mobilizing a large force, the Chinese were either beaten back or forced to abandon captured positions by the next day. Chinese documents later revealed that the ground attacks primarily served the diversionary objective, with their scales much lower than that reported by Western sources.

The PLA launched their assault on Laoshan at 05:00 on 28 April after intense artillery bombardment. PLA forces were estimated to outnumber the size of PAVN ones. The PLA 40th Division of the 14th Army crossed the border section to the west of the Lô River, while the 31st Division took Hill 1200 on the eastern bank. The Vietnamese defenders, including the PAVN 313th Division and 168th Artillery Brigade, were forced to retreat from the hills. PLA troops captured the hamlet of Na La, as well as Hills 233, 685 and 468, creating a salient of 2.5 km thrusting into Vietnam. These positions were shielded by steep cliffs covered by dense forests along the Thanh Thủy River, and could only be accessed by crossing the exposed eastern side of the Lô River valley. In roughly the same timeframe, the PLA conducted a similar operation on Zheyinshan, a nearby hill range east of Laoshan.

Fighting continued in other places such as Hills 1509 (Laoshan), 772, 233, 1200 (Zheyinshan) and 1030, over which control constantly changed hands. Although the hills were eventually secured, the performance regarding Laoshan was considered poor by the PLA. Factors such as bad timing, not considering terrain conditions and lack of flexibility between subordinates and officers lead to the assault being behind schedule with losses exceeding expectations. However unlike Laoshan, the performance for Zheyinshan was much better as the commanders allowed more flexibility and postponed the attack by an hour to let troops rest longer after heavy rain delays . Liao Xilong was the commanding officer and was promoted for his achievements.

The battle paused on 15 May, as Chinese forces had virtually secured these eight hills. Chinese records show 939 Chinese soldiers killed or wounded as well as 64 civilian laborors killed as of that date. After 15 May, the PLA started building defense fortifications on the hills.