Phosphorus is essential for agriculture and industry and is a non-renewable resource. In spite of such importance of the resource, high-grade phosphate rock deposits are rapidly being depleted and claim that economically available deposits may be exhausted entirely within a century. Therefore, finding alternative phosphorus resources is urgent for sustainable phosphorus supply. Steelmaking slag is an attractive secondary phosphorus resource from both quantity and quality viewpoints. Its advantages as alternative phosphorus include relatively high phosphorus content, sufficient supply, fairly fixed composition with slight contamination, and similar thermochemical properties as primary phosphate rock. To promote the understanding of phosphorus recovery from steelmaking slags, (1) the thermodynamic properties of phosphorus in steelmaking slags, (2) the enrichment of phosphorus-rich part from steelmaking slags, and (3) the extraction of phosphorus by pyro- and hydro-metallurgical processes are reviewed. Moreover, the economic aspects of phosphorus recovery from steelmaking slag are discussed, the approaches to promote the resource efficiency of phosphorus recovery are proposed.