Regional diplomacy serves as a foreign policy tool utilized by states towards specific regions. However, the regional diplomacy of small states within a particular region in International Relations (IR) has often received less attention and scholarly scrutiny. Drawing from neoclassical realism, this paper introduces "security needs" at the systemic level and "interest preferences" at the unitary level as primary explanatory variables. The findings reveal three main models of regional diplomacy for small states: "active regional diplomacy," "preventive regional diplomacy," and "composite regional diplomacy." Small states employ various modes of regional diplomacy to attain diverse objectives in foreign development. Analysis of the 1.0 and 2.0 periods of Mahathir's administration illustrates Malaysia's adoption of "active regional diplomacy" and "composite regional diplomacy" in cultivating relations with Northeast Asian countries. This foreign policy implementation not only fosters Malaysia's effective cooperation with nations like Japan and South Korea but also maximizes the advantageous resources of Northeast Asian countries to bolster its own development. Additionally, it steers Malaysia clear of taking sides in the intense competition among major powers, expands diplomatic avenues, and bolsters its diplomatic initiative. However, in the post-Mahathir era, the deterioration of Malaysia's relations with North Korea and tensions with China have somewhat impacted the overall stability and harmony of Malaysia's relations with Northeast Asian countries.