Interest for the gas phase interstellar silicon chemistry has been recently renewed by the detection of SST in the outer envelope of the IRC+10216 carbon star. In this contribution we present a theoretical study of the SiNH2+ molecular ion which can be seen as a precursor of silicon-nitrogen products. The radio, infrared, and electronic signatures, computed by high-level ab initio treatments using at least a 6-311++G** atomic expansion are reported. The geometry and corresponding rotational constants have been determined at the Moller-Plesset (MPn, n = 2, 3, 4), complete active space self-consistent field (CASSCF), and coupled cluster (CCSDT) levels of theory using a scaling procedure that required the evaluation of the same quantities, at the same levels of calculations, for the HNSi molecule whose rotational spectrum has been obtained experimentally. Special attention has been given to the dipole moment whose best estimated value of 0.5 +/- 0.1 D has been obtained in a series of converging treatments, including up to second-order configuration interaction. The IR spectrum, calculated at the MP2, MP3, MP4, CCSDT, and CASSCF levels, shows intense bands in the 3350 and 650 cm(-1) regions. Vibrational frequencies have been corrected using scaling factors derived from a previous study on the HNSi, HSiN, HSiNH2, and H2SiNH molecules. Rotational constants and vibrational frequencies are also provided in this report for a part of the isotopomers that can be formed upon deuteration or upon substitution by either Si-29 or Si-30 in order to facilitate future experimental interpretations or astrophysical searches. Finally, the electronic spectrum of SiNH2+. has been obtained using a coupled multiconfiguration SCF-perturbation strategy (MC/P) with an extended basis built by adding low-exponent functions to the original set : such a procedure has been shown previously to give accurate predictions for the electronic spectra of the ethylene, formaldehyde, and vinylidene molecules. In the present case, the spectrum is characterized by three intense features which should make this species observable even in low-abundance conditions.