Apostichopus japonicus is a crucial aquaculture species in Asia, and seedling production is fundamental to its cultivation. Nevertheless, the reproductive behavior of this species remains difficult to regulate artificially. Although the spawning mode of sea cucumbers is well documented, the endocrine mechanisms that regulate the synchronized processes of oocyte maturation and spawning behavior remain poorly understood. In this study, the effects of a recombinant relaxin-like gonad-stimulating peptide (RGP) on oocyte maturation and parental spawning behavior were investigated. The recombinant RGP derived from A. japonicus was effectively expressed using the Pichia pastoris system, and its biological activity was validated through mass spectrometry analysis. Results indicated that the RGP promoted oocyte maturation by inducing the rupture of germinal vesicles. Behavioral studies revealed that the RGP enhanced gamete release. Furthermore, the gametes induced by the RGP did not differ significantly from those released spontaneously in terms of progeny quality and quantity. These findings suggested that the recombinant RGP could be utilized in the breeding of A. japonicus seedlings. They also had substantial implications for elucidating the reproductive behavior of this species and for the artificial induction of seedling production.