β-Casein, a phosphoprotein representing 37% of the bovine milk caseins, has specific features promoting its application as a nanocarrier for hydrophobic bioactives. In this study, the interactions of β-casein with curcumin and vitamin D3 under the same physico-chemical conditions were investigated. The interaction kinetics have been studied by surface plasmon resonance (SPR) and fluorescence spectroscopy. The KD value for curcumin-β-casein interaction has been successfully evaluated (4.1 ± 0.7 × 10−4 M) using SPR by fitting data to a 1:1 Langmuir interaction model. Conversely, the SPR responses obtained for vitamin D3 show that the interactions between this hydrophobic compound and the β-casein immobilized on the sensor chip were below the sensitivity of the SPR apparatus. Moreover, the fluorescence quenching data show that curcumin has higher affinity to β-casein (KA = 23.5 ± 1.9 × 104 M−1) than vitamin D3 (KA = 5.8 ± 1.1 × 104 M−1).