The GDPR requires Data Controllers and Data Protection Officers (DPO) to maintain a Register of Processing Activities (ROPA) as part of overseeing the organisation’s compliance processes. The ROPA must include information from heterogeneous sources such as (internal) departments with varying IT systems and (external) data processors. Current practices use spreadsheets or proprietary systems that lack machine-readability and interoperability, presenting barriers to automation. We propose the Data Processing Catalogue (DPCat) for the representation, collection and transfer of ROPA information, as catalogues in a machine-readable and interoperable manner. DPCat is based on the Data Catalog Vocabulary (DCAT) and its extension DCAT Application Profile for data portals in Europe (DCAT-AP), and the Data Privacy Vocabulary (DPV). It represents a comprehensive semantic model developed from GDPR’s Article and an analysis of the 17 ROPA templates from EU Data Protection Authorities (DPA). To demonstrate the practicality and feasibility of DPCat, we present the European Data Protection Supervisor’s (EDPS) ROPA documents using DPCat, verify them with SHACL to ensure the correctness of information based on legal and contextual requirements, and produce reports and ROPA documents based on DPA templates using SPARQL. DPCat supports a data governance process for data processing compliance to harmonise inputs from heterogeneous sources to produce dynamic documentation that can accommodate differences in regulatory approaches across DPAs and ease investigative burdens toward efficient enforcement.