Rapid technological developments, especially in the field of social media, provide many benefits that can be enjoyed by both individuals and companies in Indonesia. Nowadays, some companies have linked themselves to social media to enhance user interactions, while in other side, people are self-directed, they can search everything in social media. Therefore, along with users who spend more time on social media will influence their purchasing decision. Consumer’s choices throughout the decision making-process and extend and leverage of the brand’s media coverage are the objective of social media marketing as part of a brand’s promotional mix. Social media has impact on sales amount, brand awareness and loyalty. This research uses a quantitative as the research design and questionnaire were distributed and the results are 122 respondents who are Instagram users and have been purchasing Nature Republic’s products in order to obtain necessary data and to test the hypothesis. Descriptive and causal research methods are used in completing this research. The result of coefficient of determination is 0.468, which means that 46.8 percent of customer purchase decision at Nature Republic Medan is influence by social media marketing. Based on the z test result, z count is higher than z table, therefore alternate hypothesis is accepted. Therefore, there is an influence of social media marketing toward customer purchase decision. The recommendation for the company based on the research result are the company must be more creative and maximize the function of promotion facilities in Instagram. Moreover, it will be good if Nature Republic Medan have its own Instagram business user.