Background The determination of cobalt in water samples (test samples) is crucial, as it is an essential metal; yet, both deficiency and excessive intake may contribute to detrimental effects in humans and animals. Objective In this study, a dispersive solid-phase extraction (DSPE) procedure was developed for the determination of trace amounts of cobalt using flame atomic absorption spectrometry (FAAS). Methods The Mn3O4-MnOOH nanocomposite was synthesized to be used as a sorbent in the preconcentration of cobalt. Results The enhancement in the detection power of the developed analytical method was quantified by the ratio of the detection limits of the Mn3O4-MnOOH-DSPE–FAAS and FAAS systems, revealing a 15-fold increase with the Mn3O4-MnOOH-DSPE–FAAS method. Conclusion The recovery results obtained in tap water for the determination of trace amounts of cobalt demonstrated the applicability and accuracy of the developed method. Highlights A sensitive analytical strategy was developed for Co. High recovery results were obtained under optimum conditions.