Using first principles calculations with the hybrid exchange–correlation functional, we systematically investigated the structural and electronic properties of silicene symmetrically and asymmetrically (Janus) decorated with halogen elements. The calculations show that all the functionalized systems are direct-band-gap semiconductors. Even more remarkable, by carefully selecting the adsorption adatoms on silicene and applying elastic tensile strain, a direct-band-gap semiconductor with any value between 0.98 and 2.13 eV can be obtained. The formation energies indicate that all the silicene derivatives should be formed experimentally. The present study suggests a rather practical way for gap opening and modulation of silicene.