HELCOM countries agreed in 2007, in cooperation with the 1991 Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS), to develop a coordinated reporting system and database for harbour porpoise observations in the Baltic Sea, including sightings, bycatch, and strandings. The dataset contains georeferenced point observations of harbour porpoise (Phocoena phocoena) reported by HELCOM Contracting Parties. Data are submitted using a standardised reporting template and compiled into the HELCOM Biodiversity database. The dataset includes both historical and contemporary records, including data originating from earlier initiatives such as the Baltic Sea Porpoise and Jastarnia projects. The compiled dataset supports regional assessment and conservation efforts under HELCOM and ASCOBANS frameworks.