We developed D2RQ Mapper (http://d2rq.dbcls.jp/), a web application to edit a mapping file of D2RQ, which is a middleware to bridge Relational Database (RDB) and Resource Description Framework (RDF). A D2RQ mapping file defines how to map data stored in an RDB to RDF in the turtle format, and to write it with a text editor is cumbersome. D2RQ Mapper assists you to edit it by contextualizing input forms in the target RDB schema and the mapping language. In addition, D2RQ Mapper supports the R2RML format to output a mapping definition. For users who need to access a target RDB within their intranets, we provide a Docker image of D2RQ Mapper.