New metastable SmSi3-x (x = 0-0.05) is obtained by high-pressure high-temperature synthesis (9.5 GPa, 870-1270 K). Powder diffraction data refinements reveal that the crystal structure of SmSi3 is isotypic to that of YbSi3 (space group I4/mmm, a = 7.23634(5) Å, c = 11.0854(1) Å). In the crystal structure, two types of Si2 dumbbells agglomerate into layers, which embed the samarium atoms. At ambient pressure, SmSi3 decomposes exothermally upon heating into Si and SmSi2-x. Single-crystal structure refinements of a specimen SmSi3-x (x=0.05) reveal considerable electron density, which is not accounted for by the YbSi3 model. The additional maxima can be assigned to disorder which affects the samarium positions and induces silicon vacancies. Scanning transmission electron microscopy experiments evidence that the disorder can be attributed to extended defects. Magnetic measurements on SmSi3-x reveal van Vleck paramagnetic behavior and antiferromagnetic ordering at low temperatures. Computations within the local spin density approximation (LSDA and LSDA+U) on the crystal structure of SmSi3 reproduce the anti­ferro­magnetic coupling as the favored long-range order. Quantum chemical analysis of the chemical bonding in SmSi3 reveals two-center two-electron bonds within the Si2 dumbbells plus a total of a little less than four electrons in lone pairs at each silicon atom.