Education in the 21st-century has developed rapidly in this advanced era. One of them is the assessment system that was previously traditional, turning into an authentic assessment. Authentic assessment aims to determine the students' ability from the affective, cognitive, and psychomotor aspects. However, there are still many obstacles in its implementation. This study aims to describe the implementation of authentic assessment in thematic learning of class IV, Islamic Elementary (MI) Superior Al Falah. This study used a qualitative approach with a case study method. The data were collected through observation, interviews, and documentation techniques. The data were analysed using Miles and Huberman's analysis technique. The results showed that the teacher implemented an authentic assessment in the thematic learning. Such assessment could be viewed from three aspects, namely the planning, the implementation, and the processing of the assessment results. The planning stage was done by compiling the lesson plans with the assessment instruments. The assessment instruments included an observation sheet, a written test, a project, and performances. In the implementation stage, some of the assessments were not in accordance with the design made. The processing of the results of the authentic assessment was carried out at the end of each semester.