With the rapid development of miniaturized underwater observation platforms, including wearable devices, micro-robots, and animal-borne tags, there is an increasing demand for compact and low-power conductivity sensors for salinity measurement. Conventional conductivity measurement systems typically rely on a multi-stage architecture involving AC excitation, signal conversion, analog conditioning, and high-precision analog-to-digital conversion, resulting in bulky hardware, high power consumption, and limited system integration. Although several low-power integrated approaches have been reported, their measurement accuracy is generally compromised.To address these challenges, this study proposes a novel miniaturized frequency-output conductivity measurement system based on the self-excited oscillation principle. Seawater is incorporated into a self-oscillating loop as a variable resistance, allowing conductivity variations to be directly converted into frequency changes. By integrating excitation, detection, and conversion within a single oscillation loop, the proposed “3-in-1” architecture replaces the conventional DAC–instrumentation amplifier–ADC measurement chain and substantially simplifies the core hardware.The measurement system has a compact size of Ø40 mm × 10 mm, an operating power consumption of 27.12 mW, and a conductivity measurement range of 0–77 mS/cm. Integrated digital temperature and pressure sensors provide compensation for environmental variations. In an in-situ sea trial over a 0–60 m depth profile, the system exhibited a maximum absolute conductivity deviation of 0.123 mS/cm compared with a reference SBE 917 CTD after stabilization. In addition, a 36-day immersion test in artificial seawater demonstrated a salinity drift rate of less than 0.005 PSU per day after temperature and pressure compensation.The proposed system provides a compact, low-power, and highly integrated solution for conductivity and salinity sensing, offering a promising technical pathway for resource-constrained marine robotic platforms, underwater wearable devices, biological tagging, and long-term ocean profile observations.