Provided are: a nucleating agent capable of simultaneously improving the thermal stability and the transparency of a polyolefin resin; a polyolefin-based resin composition containing the same; and a molded article of the composition. The nucleating agent contains a compound represented by Formula (1) below where R1 to R4 each independently represent a hydrogen atom or a linear or branched alkyl group having 1 to 9 carbon atoms; R5 represents an alkanediyl group having 1 to 4 carbon atoms; and M represents a hydrogen atom or a sodium atom. In this nucleating agent, the mass ratio of phosphorus atoms and sodium atoms, P/Na, is in a range of 1.200 to 1.500.