In subsea applications, there is a growing demand for high-speed wireless communication links for transmitting data between different equipment. Radio communication is constrained by the high attenuation in seawater. Only a very short range is achievable, even at low frequencies. In this thesis an independent, battery-driven radio frequency transmitter is developed and tested to investigate the properties of, and prove the concept of underwater radio communication. The transmitter is made on the basis of a cNODE Research Midi system from Kongsberg Maritime. A software-defined radio receiver is used to measure and store the received signal for post-processing. BPSK modulated data is transmitted through seawater with a 5 MHz carrier with several data rates from 237 bps to 46.598 kbps. Two tests with approximately 50 m and 11 m depths in Trondheimsfjorden have been conducted. Maximum range was measured to be close to 5 m at 9 m depth with 1 W (+30 dBm) output power. Although, this is believed to be in an area where there might be a lot of freshwater mixed in the seawater, which would in theory improve the range compared to for pure seawater. Data from the software-defined radio has been successfully demodulated with a custom made MATLAB program for all data rates, hence proven the underwater radio communication concept.