β-(1→4)-Mannobiose (Man₂) is an attractive oligosaccharide for food and feed additives. Quantifying Man₂ is useful for the industrial production of Man₂. In this study, we evaluated colorimetric and HPLC-based quantification methods for Man₂. For the colorimetric methods, quantification of d-glucose and α-d-mannose 1-phosphate (Man1P) was combined with the epimerization of Man₂ to Manβ-4Glc and phosphorolysis of Manβ-4Glc. For the HPLC-based methods, high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and ultra-performance liquid chromatography with P-aminobenzoic ethyl ester labeling (ABEE-UPLC) were applied. The Man1P-based colorimetric method and HPAEC-PAD accurately determined Man₂ even in the presence of other sugars tested. The d-glucose-based colorimetric method was unsuitable for samples containing excess concentrations of d-glucose and d-mannose, and the ABEE-UPLC method was unsuitable for samples containing cellobiose and maltose. For quantifying Man₂ in mannanase-treated copra meal extract, the analytical values from the four methods corresponded well with each other.