The K9 Thunder is a South Korean 155 mm self-propelled howitzer designed and developed by the Agency for Defense Development and private corporations including Samsung Aerospace Industries, Kia Heavy Industry, Dongmyeong Heavy Industries, and Poongsan Corporation for the Republic of Korea Armed Forces, and is now manufactured by Hanwha Aerospace. K9 howitzers operate in groups with the K10 ammunition resupply vehicle variant.

The entire K9 fleet operated by the ROK Armed Forces is now undergoing upgrades to K9A1, and a further upgrade variant K9A2 is being tested for production. As of 2022, the K9 series has had a 52% share of the global self-propelled howitzer market, including wheeled vehicles, since the year 2000.

In the 1980s, the ROK Armed Forces came in need of a new artillery system to contest North Korean equipment. The armed forces operated M107 self-propelled guns and K55 self-propelled howitzers. However, they had shorter firing ranges compared to M-1978 Koksan and were outnumbered by various North Korean artillery. With the success in designing and manufacturing the KH178 105 mm and KH179 155 mm towed howitzers, and experience gained by license producing the K55 (KM109A2), the Ministry of Defense ordered the development of a new system that would have a longer firing range, faster firing rate, and high mobility. The development started in 1989 and was led by the Agency for Defense Development (ADD) and Samsung Aerospace Industries (now Hanwha Aerospace).

Since 1983, the ADD researchers have been collecting and analyzing data for future artillery. They saw that burst fire and quick relocation would become the dominant factor in artillery battles and built an automatic loading system for testing in 1984. In 1987, the ADD offered an upgrade plan to the existing K55 inspired by the United States' M109 Howitzer Improvement Program (standardized in 1990 as M109A6), but was rejected by the Republic of Korea Army in 1988. As a result, and at the beginning of K9 development, the ADD was determined to create a new weapon system and worked on a conceptual model until 1991. Early concepts requested by the military included river crossing capability and the installation of M61 Vulcan as an anti-air weapon, which were later removed due to being unnecessary for such a long-range weapon.

In 1989, the only design data the researchers could obtain at that time were the provisions of the four-nation ballistic agreements, namely the United States, the United Kingdom, Germany, and Italy, to secure the homogeneity of the ammunitions—the new 52 caliber gun fires a NATO standard ammunition (L15A1) at a speed of 945 m per second from chamber volume of 3,556 cm³. The first domestic design was prepared by extending and modifying the 155 mm 39 caliber gun used for the KH179. The first firing test was held in January 1992 but experienced many problems due to design errors.

In September 1990, a Korean developer visited the United Kingdom in search of turret design technology which was known to have been developed by British Vickers for AS-90. Vickers refused the technology transfer. Instead, the company offered the AS-90 for sale. The developer also visited Marconi, but the negotiations ended with an unsatisfactory result due to a high requested price. Therefore, developers pursued a domestic electrohydraulic driving system, using the experience in turret design and turret driving devices of the K1 MBT. The simulator was built in 1991.

The next year, researchers found that the disproportionate moment of the 52-caliber was twice of the K55. The balancing machine, which increased the capacity of the existing hydraulic balancing machine, did not sufficiently compensate for the imbalance moment value due to the change in the position of the armament. The driving force was very different depending on the driving angle. The same problem appeared in Germany's Panzerhaubitze 2000, which was under development. A joint research team from the ADD and Seoul National University of Science and Technology calculated an accurate theoretical model, and concluded that adjustment to system configuration was possible without major design changes.

Meanwhile, loud noise from hydraulic generators, which can cause hearing loss under long exposure, was also a problem. The engineers of the ADD and Dongmyeong Heavy Industries (now Mottrol) found that the noise was due to excessive shaking of hydraulic pressure, thus creating an experimental device using the principle of Helmholtz attenuators used in car mufflers. The noisy equipment became quieter and the hydraulic pulsation was significantly reduced. Overall, the domestic design showed driving precision of less than 1 mil in the standard error range.