Abstrak Sport ecotourism adalah kegiatan yang memadukan aktivitas olahraga dan pariwisata. Trail running merupakan salah satu sport ecotourism yang saat ini banyak diminati berbagai kalangan. Kabupaten Tuban Jawa Timur memiliki potensi besar untuk pengembangan sport ecotourism. Desa Ngrejeng Kecamatan Grabagan Kabupaten Tuban memiliki potensi untuk dikembangkan menjadi salah satu destinasi tujuan sport ecotourism karena wilayahnya terdiri dari perbukitan karst yang memiliki kontur tidak rata sehingga menimbulkan tantangan bagi pelari. Selain itu, bentang alam ekosistem karst yang unik dan menarik dapat memberikan pengalaman bagi pelari. Oleh karena itu, kegiatan trail running bertajuk Ngrejeng Karst Running bukan hanya memberikan dampak kesehatan raga juga meningkatan kesehatan jiwa. Tujuan kegiatan pengabdian kepada masyarakat ini adalah untuk memberikan pendampingan kepada karang taruna Desa Ngrejeng Kecamatan Grabagan untuk menyelenggarakan event olahraga berupa trail running. Trail running bertajuk Ngrejeng Karst Running ini melintasi 3 Desa yaitu Desa Grabagan, Desa Banyubang dan berakhir di Desa Ngrejeng. Metode pelaksanaan kegiatan ini dilakukan melalui 3 tahap yaitu: 1) tahap sosialisasi, 2) tahap pendampingan penyelenggaraan trail running event yang meliputi berbagai macam kegiatan, 3) tahap monitoring dan evaluasi. Hasil dari kegiatan ini adalah terbentuk tim penyelenggara (panitia) kegiatan, penentuan rute lari, berhasil menyusun proposal sponsorship dan promosi event trail running di berbagai platform media sosial. Berdasarkan hasil kegiatan pengabdian kepada masyarakat ini diharapkan mitra secara mandiri maupun kolaborasi dengan tim PKM mampu menyelenggarakan berbagai macam event terkait sport ecotourism secara berkelanjutan guna mendukung upaya pembangunan desa melalui pengembangan ekowisata. Kata kunci: karang taruna; karst; ngrejeng; trail running. Abstract Sport ecotourism is an activity that combines sports and tourism. Trail running is one of the most popular forms of sport ecotourism that is currently gaining traction among various groups. Tuban Regency in East Java has significant potential for the development of sport ecotourism. Ngrejeng village has the potential to be developed into sport ecotourism destination because its terrain consists of karst hills with uneven contours, presenting a challenge for runners. In addition, the unique karst ecosystem landscape can provide an experience for runners. Therefore, the trail running activity entitled Ngrejeng Karst Running not only provides physical health benefits but also improves mental health. The purpose of this community service activity is to provide assistance to the Ngrejeng Village youth organization to organize a trail running event. The Ngrejeng Karst Running event traverses three villages: Grabagan, Banyubang, and Ngrejeng. The implementation of this activity is carried out in three stages: 1) the socialization stage, 2) the support stage for organizing the trail running eventand 3) the monitoring and evaluation stage. The outcomes of this activity include the formation of an organizing committee for the event, the determination of the running route, the sponsorship proposal, and the promotion of the trail running event. Based on the results of this community service activity, it is hoped that partners, either independently or in collaboration with the PKM team, will be able to organize various events related to sport ecotourism in a sustainable manner to support village development efforts through the development of ecotourism. Keywords: karst; ngrejeng; trail running.