In many countries, energy diplomacy is usually carried out by the national government as the primary unit, through bilateral or multilateral agreements. However, it seems that Surabaya and Kitakyushu’s cities have carried out intensive cooperation in the energy sector since the 2010s. This condition shows that the implementation of energy diplomacy has also been conducted at the substate level. Even though it focuses on the active role of the two cities, the meaning of energy diplomacy has not disappeared, but instead, it has expanded. This paper analyzes the City of Surabaya, Indonesia, approaches toward the City of Kitakyushu, Japan, in developing the energy sector, especially to strive for an energy-efficient city planning system in Surabaya. This paper’s analytical framework is based on energy diplomacy and diplomacy by sub-national actors in international relations. The finding of this study is that the diplomacy carried out by the City of Surabaya toward Kitakyushu is classified as a practice of energy diplomacy. The diplomacy is carried out through a sister city approach.

Summary Energy diplomacy is usually conducted by national governments. However, the case of sister city co-operation between the cities of Surabaya, Indonesia, and Kitakyushu, Japan, shows how substate actors can perform energy diplomacy by developing technology to create public spaces that apply energy efficiency and energy-saving principles. This article offers a new angle on energy diplomacy by elaborating on the role of the city government. To future-proof our perspective, we applied qualitative methods by gaining data from in-depth interviews and focus group discussions, then triangulated the result from the literature about energy diplomacy. The article argues that energy diplomacy has expanded in the sense that it is now carried out at municipal as well as national level and yet still adheres to states’ foreign policy agendas in the energy sector. These substate actors ultimately strengthen the principle of energy utilisation that has been regulated at the national level within the framework of bilateral co-operation with other substate actors.