The dose enhancement effect of various paramagnetic nanoshells irradiated with various brachytherapy sources was calculated. 125I, 103Pd, hypothetical 169Yb, and 192Ir brachytherapy sources were simulated using the MCNPX Monte Carlo code. The dose rate constants and radial dose functions of the sources were calculated and compared with previously published data. Au/Gd, Au/Fe2O3, Ag/Dy, and Ag/Mn nanoshells with concentrations of 10, 20, and 30 mg/ml in tumor tissue were simulated. A soft-tissue phantom containing a 1 cm × 1 cm × 1 cm tumor was loaded with nanoshells. The dose enhancement factor (DEF) was calculated in voxels inside and outside the tumor. The highest average DEFs in tumor tissue were observed with the 125I source for the 30 mg/ml tumor concentration, with DEF values for Au/Gd, Au/Fe2O3, Ag/Dy, and Ag/Mn nanoshells of 3.25, 2.82, 2.86, and 2.77, respectively. The calculated dose rate constant and radial dose function of the sources were in good agreement with previously published data. Based on our results, nanoshells can be utilized in brachytherapy to enhance the tumor dose and because of their biocompatible shells, their toxic effects can be avoided. The nanoshells are paramagnetic and can be applied simultaneously in brachytherapy and magnetic resonance imaging. However, clinical application of nanoshells in brachytherapy requires more preclinical studies.