We have calculated electric dipole polarizabilities of the 3s(2) S-1(0), 3s3p P-3(0), and 3s3p P-1(1) states of the Si2+ ion using the recently developed configuration interaction + all-order method. A detailed evaluation of the uncertainties of the final results is carried out. Our value for the ground-state electric dipole polarizability 11.670(13) a.u. is in excellent agreement with the resonant excitation Stark ionization spectroscopy value 11.669(9) a.u. [Komara et al., J. Phys. B 38, 87 (2005); Mitroy, Phys. Rev. A 78, 052515 (2008)]. This paper represents a benchmark test of theory and experiment in divalent atoms. The near cancellation of the ns(2) S-1(0) ground state and the lowest nsnp P-3(0) polarizabilities previously observed in B+, Al+, In+, Tl+, and Pb2+ is also found in the Si2+ ion.