Learning mathematics which is carried out conceptually, usually make students feel bored, less interested, less creative, their abilities do not develop, and mathematics learning achievement has so far not been satisfactory. Efforts to improve mathematics learning have been carried out for a long time in various ways and objectives, one of which is the implementation of the Kurikulum Merdeka. One of the lessons that can be carried out to support the implementation of the Kurikulum Merdeka is Realistic Mathematics Education so that a study is carried out by implementing Realistic Mathematics Education in class X BD students at SMK Negeri 1 Pujer with the theme of Online Lending. The research desian is classroom action research using a qualitative approach with one cycle consisting of (1) Planning, (2) Action implementation, (3) Observation, and (4) Reflection. From the results of the study it was concluded that students' understanding of mathematics subject matter of single interest and compound interest with the theme of Online Lending increased. This is evidenced by the activeness of students during discussions, the results of simulations and accurate student calculations and students' interest in learning about the advantages and disadvantages of Online Lending other than what has been discussed.