In recent decades, research became more data-intensive in the fast-paced information environment. Researchers are facing new challenges in managing their research data due to the increasing volume of data-driven research and the policies of major funding agencies. Information professionals have begun to offer various data support services such as training, instruction, data curation, data management planning and data visualization. However, the emerging field of data librarians, including specific roles and competencies, has not been clearly established even though librarians are taking on new roles in data services. Therefore, there is a need to identify a set of competencies for data librarians in this growing field. The purpose of this study is to consider varying core competencies for data librarians. This exploratory study examines 95 online recruiting advertisements regarding data librarians posted between 2017 and 2021. This study finds core competencies for data librarians that include skills in technology, communication and interpersonal relationships, training/consulting, service, library management, metadata knowledge and knowledge of data curation. Specific core technology skills include knowledge of statistical software and computer programming. This study contributes to an understanding of core competencies for data librarians to help future information professionals prepare their competencies as data librarians and the instructors who develop and revise curriculum and course materials.