The Third Front Movement (Chinese: 三线建设; pinyin: Sānxiàn jiànshè) or Third Front Construction was a Chinese government campaign to develop industrial and military facilities in the country's interior. The campaign was motivated by strategic depth concerns that China's existing industrial and military infrastructure would be vulnerable in the event of invasion by the Soviet Union or air raids by the United States. The largest development campaign of Mao-era China, it involved massive investment in national defense, technology, basic industries (including manufacturing, mining, metal, and electricity), transportation and other infrastructure investments and was carried out primarily in secret.

The Third Front is a geo-military strategy, ensuring the country greater defense in depth: it is relative to the "First Front" area that is close to the potential war fronts. The Third Front region covered 13 provinces and autonomous regions with its core area in the Northwest (including Shaanxi, Gansu, Ningxia, and Qinghai) and Southwest (including today's Sichuan, Chongqing, Yunnan, and Guizhou). Its development was motivated by national defense considerations following the escalation of the Vietnam War after the Gulf of Tonkin Incident, the Sino-Soviet Split and small-scale border skirmishes between China and the Soviet Union.

The Third Front campaign industrialized part of China's rugged interior and agricultural region. Between 1964 and 1980, China invested 205 billion yuan in the Third Front Region, accounting for 39.01% of total national investment in basic industries and infrastructure. Millions of factory workers, cadres, intellectuals, military personnel, and tens of millions of construction workers, flocked to the Third Front region. More than 1,100 large and medium-sized projects were established during the Third Front period. With large projects such as Chengdu-Kunming Railway, Panzhihua Iron and Steel, Second Auto Works, the Third Front Movement stimulated previously poor and agricultural economies in China's southwest and northwest. Dozens of cities, such as Mianyang, Deyang, and Panzhihua in Sichuan, Guiyang in Guizhou, Lanzhou and Tianshui in Gansu, and Shiyan in Hubei, emerged as major industrial cities.

However, the designs of many Third Front projects were uneconomic due to their location or deficient due to their hurried construction. For national defense reasons, location choices for the Third Front projects followed the guiding principle "Close to mountains, dispersed, hidden". Many Third Front projects were located in remote areas that were hard to access and far away from supplies and potential markets. The Third Front Movement was carried out in a hurry. Many Third Front projects were simultaneously being designed, constructed, and put in production.

After rapprochement with the United States reduced the national defense considerations underlying the Third Front, investment in its projects decreased. Since the reform of state-owned enterprises starting in the 1980s, many Third Front plants went bankrupt, though some others reinvented themselves and continued to serve as pillars in their respective local economies or were developed into successful private enterprises.

Mao created the concept of the Third Front to locate critical infrastructure and national defense facilities away from areas where they would be vulnerable to invasion,. thereby ensuring greater military defense in depth. Describing the geographical foundation of the concept, he stated:

Our first front is coastal regions, second front is the line that cuts from Baotou to Lanzhou and southwest is the third front ... in the period of the atomic bomb, we need a strategic rear for retreat, and we should be prepared to go into the mountains [to become guerilla]. We need a place like this.

The "Big Third Front" included the Northwest and Southwest provinces like Qinghai, Sichuan, Guizhou, and Yunnan. In comparison, the "First Front" was composed of the major cities from Manchuria down to the Pearl River Delta and the "Second Front" referred to the smaller cities located further inland from the First Front.