Marine ecosystems have been under pressure for decades as a result of intensive human activity. Cetaceans have also been negatively affected by our actions, so to implement protection and conservation plans, it is crucial to know the current state of the populations. In the last decades, passive acoustic monitoring (PAM) has become an important tool to fulfil this purpose. This study, makes use of PAM to investigate the spatial and temporal occurrence of cetaceans in the Norwegian Skagerrak between March 2023 and February 2024. Two methods were used: i) Monthly transect ship surveys from Norway to Denmark using a towed hydrophone array; and ii) Moored Continuous Porpoise Detectors (CPODs, n = 6) deployed at the Norwegian coast to monitor the presence and the vocal activity of harbour porpoises (Phocoena phocoena). Towed hydrophone data were analysed using the PAM software PAMGuard to study the cetacean occurrence along the year. The CPOD data were used to investigate spatial, seasonal and diel patterns in porpoise presence and vocal activity, as well as how sea temperature and currents affect these. Ship surveys revealed the presence of harbour porpoises, northern bottlenose whales (Hyperoodon ampullatus), minke whales (Balaenoptera acutorostrata) and bottlenose dolphins (Tursiops truncatus) in the area. Generalized Additive Models, fitted to the CPOD data, showed spatial, seasonal and diel variations on harbour porpoise presence and vocal activity, while the effect of hydrographic conditions was less clear. Significantly higher harbour porpoise presence and vocal activity were registered during spring, especially in the northernmost areas of Raet National Park. Moreover, porpoise presence was significantly higher during the night, independently of the time of the year. Changes in prey availability, as well as porpoise calving and mating seasons, and prey migration may be behind these observations. This study was the first of its kind performed in the Norwegian Skagerrak, and it provided novel information about specially, the harbour porpoises in the Norwegian Skagerrak. However, important knowledge gaps must still be filled to properly manage them.