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Traditional metal chelators, such as ethylenediaminetetraacetic acid (EDTA), have been gradually replaced due to their poor biodegradability in soil and high risk of heavy metal leaching into groundwater, which pose high environmental risks to the health of humans and animals. In this study, a liquid amino acid fertilizer (LAAF, waste proteins from hydrolysates of animal carcasses) and polyaspartate (PASP) were used as additives to enhance the phytoextraction of cadmium (Cd) and lead (Pb) from contaminated soil. We conducted pot experiments to investigate the phytoextraction capacity of Solanum nigrum, a Cd accumulator, grown on soil highly contaminated with Cd and Pb in the absence (as controls) or presence of PASP and LAAF. Both PASP and LAAF significantly improved plant growth, Cd accumulation, and total Cd and Pb content in S. nigrum shoots and roots. PASP and LAAF application promoted Cd translocation from roots to shoots in S. nigrum and Cd bio-accessibility in rhizosphere soils, but this was not the case for Pb. Both PASP and LAAF increased Cd and Pb phytoextraction by S. nigrum plants, and Cd phytoextraction was more effective in LAAF-assisted S. nigrum than in PASP-assisted S. nigrum. These findings demonstrate that the low cost and ecofriendly features of recycled waste proteins make them good candidates for chelant-enhanced phytoextraction from heavy metal-contaminated soils.