Tianhe-2 or TH-2 (Chinese: 天河-2; pinyin: tiānhé-èr; lit. 'Heavenriver-2', i.e. 'Milky Way 2') is a 33.86-petaflop supercomputer located in the National Supercomputer Center in Guangzhou, China. It was developed by a team of 1,300 scientists and engineers.

It was the world's fastest supercomputer according to the TOP500 lists for June 2013, November 2013, June 2014, November 2014, June 2015, and November 2015. The record was surpassed in June 2016 by the Sunway TaihuLight. In 2015, plans by Sun Yat-sen University in collaboration with Guangzhou district and city administration to double its computing capacities were stopped by a U.S. government rejection of Intel's application for an export license for the CPUs and coprocessor boards.

In response to the U.S. sanctions, China introduced the Sunway TaihuLight supercomputer in 2016, which substantially outperforms the Tianhe-2 (and also affected the update of Tianhe-2 to Tianhe-2A, replacing U.S. tech), and in November 2022 ranks eighth in the TOP500 list while using completely domestic technology including the Sunway manycore microprocessor.

The development of Tianhe-2 was sponsored by the 863 High Technology Program, initiated by the Chinese government, the government of Guangdong province, and the government of Guangzhou city. It was built by China's National University of Defense Technology (NUDT) in collaboration with the Chinese IT firm Inspur. Inspur manufactured the printed circuit boards and helped with the installation and testing of the system software. The project was originally scheduled for completion in 2015, but was instead declared operational in June 2013. As of June 2013, the supercomputer had yet to become fully operational. It was expected to reach its full computing capabilities by the end of 2013.

In June 2013, Tianhe-2 topped the TOP500 list of fastest supercomputers in the world and was still listed as the fastest machine in the November 2015 list. The computer beat out second-place finisher Titan by nearly a 2-to-1 margin. Titan, which is housed at the United States Department of Energy's Oak Ridge National Laboratory, achieved 17.59 petaflops, while Tianhe-2 achieved 33.86 petaflops. Tianhe-2's performance returned the title of the world's fastest supercomputer to China after Tianhe-I's début in November 2010. The Institute of Electrical and Electronics Engineers said Tianhe-2's win "symbolizes China's unflinching commitment to the supercomputing arms race". In June 2013, China housed 66 of the top 500 supercomputers, second only to the United States' 252 systems. The Chinese total increased to 168 of the top 500 systems by June 2016, overtaking the United States which fell to 165 of the top 500 supercomputers.

Graph500 is an alternate list of top supercomputers based on a benchmark testing analysis of graphs. In their benchmark, the system tested at 2,061 gigaTEPS (traversed edges per second). The top system, IBM Sequoia, tested at 15,363 gigaTEPS. It also holds first place in the HPCG benchmark test proposed by Jack Dongarra, with 0.580 HPCG PFLOPS in June 2014.

Tianhe-2 has been housed at National University of Defense Technology.

According to NUDT, Tianhe-2 would have been used for simulation, analysis, and government security applications.