This article proposes and introduces the design and implementation of a novel bidirectional acoustic tag (BTAG) for marine biotelemetry, enabling enhanced tracking and data collection from aquatic species. Unlike conventional unidirectional tags, the BTAG supports bidirectional communication, allowing for precise time-of-flight (ToF) measurements and improved geolocation accuracy. This system reduces deployment costs and the post-processing complexity while ensuring compatibility with existing acoustic telemetry networks. The BTAG employs ultralow-power components with a one-year bidirectional life and a ten-year unidirectional mode, with a miniaturised footprint of 50 mm in length and 17 mm in diameter, and a tailored piezoelectric transducer, achieving operational longevity despite its miniaturised form factor. Field and laboratory tests demonstrate its performance, achieving effective transmission ranges up to 250 m at an 80% reception rate, and optimal reception capabilities, such as 70 m at sea, with 90 m tested in freshwater. The system holds the potential for integration with autonomous vehicles, further advancing marine research and conservation applications.