The study incorporates the urea detection in non-enzymatic urea samples and enzymatic urea-urease samples utilizing the phenomenon of surface plasmon resonance (SPR)/localized surface plasmon resonance (LSPR) realized on the core of a multimoded optical fiber. The designed bio-sensor, primarily a refractive index sensor well operates for the urea detection in the concentrations ranging 50–800 mM. The experimental results show that as urea concentration increases in both samples (non-enzymatic and enzymatic), refractive indices also increases leading to a red shift in the resonance wavelength (SPR wavelength). Further, the sensing probe was modified by coating biosynthesized gold nanoparticles (AuNPs) on to the thin layer of gold (Au) by dipping the probe in the solution of biosynthesized AuNPs. After performing the experiments, the sensitivity values were calculated for solutions of different concentration of urea in non-enzymatic and enzymatic urea samples and compared with the probes coated with Au film only. A comparative study of the two set of results reveals a two-fold and three-fold enhancement in the sensitivity values for non-enzymatic and enzymatic urea samples respectively as compared with the probe without AuNPs. Authors believe that the present fiber optic bio sensing probe is easy to design, low cost, capable of remote sensing. Also it works well in the range near to the pathophysiological range of urea in human blood and hence can be beneficial for the determination of urea concentration in medical diagnostics as well.