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Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBiSe/BiSe heterostructure, which was fabricated by self-assembling a MnBiSe layer on top of the BiSe surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBiSe/BiSe heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future >topotronics> devices. ; This work has been supported by Grants-In-Aid from Japan Society for the Promotion of Science (Nos. 15H05453, 16K13683, 19340078, and 23244066), the Toray Science Foundation, the Basque Country Government, Departamento de Educacion, Universidades e Investigacion (Grant No. IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos. FIS2010-19609-C02-01, FIS2013- 48286-C02-02-P, and FIS2013-48286-C02-01-P), the Tomsk State University Academic D.I. Mendeleev Fund Program (Grant No. 8.1.05.2015), and Saint Petersburg State University (project 15.61.202.2015). The ARPES experiments were performed under the UVSOR Proposal Nos. 25-808, 26-531, 27-533, 28-526, and S-15-MS-0034, and the SARPES experiments were performed under the HiSOR Proposal No. 15-A14. The XMCD measurements were performed under the UVSOR proposal number S-16-MS-2017. The LEED measurements were performed under the ISSP Proposal number H17-A250. The SQUID measurements were performed using facilities of the Cryogenic Research Center, the University of Tokyo. ; Peer Reviewed