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Unlike time-reversal topological insulators, surface metallic states with Dirac cone dispersion in the recently discovered topological crystalline insulators (TCIs) are protected by crystal symmetry. To date, TCI behaviors have been observed in SnTe and the related alloys PbSnSe/Te, which incorporate heavy elements with large spin-orbit coupling (SOC). Here, by combining first-principles and ab initio tight-binding calculations, we report the formation of a TCI in relatively lighter rocksalt SnS and SnSe. This TCI is characterized by an even number of Dirac cones at the high-symmetry (001), (110), and (111) surfaces, which are protected by the reflection symmetry with respect to the (10) mirror plane. We find that both SnS and SnSe have an intrinsically inverted band structure even without the SOC and the SOC is necessary only to open the bulk band gap. The bulk band gap evolution upon volume expansion …