Single crystal of Tris-allylthiourea mercury bromide (ATMB), a NLO active complex of allylthiourea was grown by slow evaporation technique at 305 K. The structure of the grown crystal was confirmed by single crystal XRD. The composition of the sample was identified by CHN elemental analysis. The optical properties were investigated by FTIR and optical absorption spectroscopic techniques. From the optical absorption spectrum, the cut-off wavelength and energy gap of the sample were found to be 350 nm and 3.9280 eV respectively. The SHG efficiency of ATMB was found to be nearly 3.5 times higher than that of urea. Thermal analysis confirms that the crystal is thermally stable up to 125.11 °C. Scanning Electron Microscope (SEM), photoluminescence (PL), dielectric and dc conductivity studies were carried out and reported for the first time.