Karangsambung-Karangbolong National Geopark (KKNG) is located in Kebumen, Central Java, Indonesia, and was designated as such in 2018 by the Indonesian Ministry of Energy and Mineral Resources, based on recommendations from the Indonesian National Geopark Committee (INGC). The INGC made many recommendations before proposing KKNG as a candidate to become a United Nations Educational, Scientific and Cultural Organization Global Geopark (UGGp), for example, proposing the main theme of KKNG. This study aims to identify geodiversity, and assess and analyze geosites to determine the history and geological evolution of the geopark area, which are used to determine the geopark's theme. Geosite assessment methods using government regulatory standards in Indonesia demonstrated that KKNG has 41 geological sites: 29 geosites are in the northern area and 12 geosites are in the southern area. Furthermore, 12 geosites have high values (301–400), and 28 geosites have moderate values (201–300). Based on the assessment and identification, the northern part of the KKNG subtheme is the ancient oceanic floor and subduction zone, and the southern part is a conical karst landscape. The proposed main theme geopark is the best evidence of plate tectonic theory in Southeast Asia and karstic landscape, with its primary mission to realize a sustainable global geopark for the welfare of society.