The KAI KF-21 Boramae (Korean: KF-21 보라매; lit. 'KF-21 Hunting Hawk'; formerly KF-X; commonly KF-21) is a South Korean twin-engine fighter aircraft. The initial goal of the program was to develop an indigenous multirole fighter for the Republic of Korea Air Force (ROKAF). The airframe of Blocks I and II adopts semi-stealth technology with partially embedded external weapons and MUM-T (Manned-Unmanned Teaming) capabilities, while full stealth, including internal weapons bays is planned for the Block III or KF-21EX variant. As South Korea's second domestic fighter jet development program following the FA-50 series, the KF-21 makes the nation the fourth in the world—joining the United States, China, and Russia—to produce an aircraft featuring stealth avionic architecture.

The Block I program is led by the South Korean government, which holds 60% of the shares. An additional 20% is held by the manufacturer Korea Aerospace Industries (KAI), with Indonesia holding the final 20% stake. Later, in August 2024, Indonesia's stake was reduced to 7.5% due to a request from the Indonesian government. The Block II program, which focuses primarily on the serial integration of air-to-ground and air-to-ship armaments, will be executed alongside new international partners through early 2027. This collaborative approach will extend into the Block III (KF-21EX) program, which is also set to be carried out with newly formed global partnerships.

In April 2021, the first prototype was completed and unveiled during a rollout ceremony at the headquarters of KAI at Sacheon Airport. It was named the Boramae after a juvenile Eurasian goshawk. The maiden flight was on 19 July 2022. Serial production started in July 2024. The initial rollout took place in March 2026. 40 Block I units are scheduled for deployment beginning in September 2026; the ROKAF expects to complete additional 80 Block II units deployment by 2032. It will also be available for export. The ROKAF will initially begin replacing its aging F-4D/E Phantom II and F-5E/F Tiger II jets with the KF-21. Over time, the KF-21 will also replace its fleet of KF-16s and F-15Ks in service.

Announced in March 2001 by South Korean President Kim Dae-jung at a graduation ceremony of the Korea Air Force Academy, the KF-X project aimed to produce an advanced multirole jet fighter to replace South Korea's aging F-4D/E Phantom II and F-5E/F Tiger II aircraft. In 2002, research and development (R&D) requirements were determined by the Joint Chiefs of Staff. The project was considered extremely ambitious; the Korea Institute for Defense Analyses (KIDA), a state-owned defense ministry think tank, expressed serious doubt about the country's ability to complete such a complex undertaking. The ROK Air Force expressed concerns that a delay or failure of the KF-X program could significantly compromise its future security readiness. During this period, the project faced heavy scrutiny from local media, which maintained a predominantly pessimistic tone regarding its massive budget and technical viability.

Consequently, the program was subjected to a grueling cycle of ten separate feasibility studies, leaving the evaluation phase fraught with delays while its economic cost and potential risks were debated. However, the project received renewed interest after a feasibility study in 2008 and attacks by North Korea in 2010. The last feasibility study concluded that developing the domestic defense industry was considered a matter of national importance and expected to generate positive ripple effects across high-tech sectors despite the project's risks and the significantly higher per-unit cost compared to foreign alternatives. The study still required a foreign partner to mitigate financial risks.

To fulfil the foreign partnership requirement, South Korea first offered Turkey a 20% stake in the KF-X project; however, Turkey declined, seeking greater control over the initiative. The South Korean government expected to commit 60% of the development cost, with the remaining 40% to be covered by domestic and foreign partners. As an alternative, Indonesia agreed to fund the 20% share through Indonesian Aerospace and purchase 50 of the approximately 150–200 planned aircraft. On 15 July 2010, South Korea and Indonesia formalised a partnership, launching the KF-X development program with a target to begin aircraft deliveries in 2026.

The initial goal for the program was to develop a single-seat twin-engine multirole fighter with stealth capabilities exceeding the Dassault Rafale, Eurofighter Typhoon, F-16 Fighting Falcon, but less than those of the Lockheed Martin F-35 Lightning II. The Weapon Systems Concept Development and Application Research Center of Konkuk University advised that the KF-X should be superior to the F-16 Fighting Falcon, with 50% greater combat range, 34% longer airframe lifespan, better avionics, active electronically scanned array (AESA) radar, more-effective electronic warfare, and data link capabilities. Their recommendations specified approximately 50,000 pounds-force (220,000 N) of thrust from two engines, supersonic interception and cruising capabilities, and multi-role capabilities. The Block I configuration requirements were later finalised by the ROKAF and DAPA to a 4.5 generation fighter with semi-stealth capabilities.

At the onset of configuration development, South Korea had achieved a 65% indigenous technology localization rate for the KF-X program, prompting efforts to both develop the remaining capabilities domestically and seek international partnerships to fill the gaps. The Agency for Defense Development (ADD) proposed two primary KF-X concepts: C103, resembling the F-35, and C203, modelled after European fighters with forward canards. These configurations were outlined for potential partnership with the United States or European partners.