An ultrahigh-performance engineered cementitious composite (UHP-ECC) is a cementitious composite with both high compressive strength and tensile ductility, which has been regarded as a promising strengthening material for the retrofitting of existing concrete structures. Thus, the bond behavior between UHP-ECC and old concrete has become a concern because the interface is inevitably subjected to shear and tensile loads. To this end, this paper intends to investigate the shear and splitting tensile properties of the interface between UHP-ECC and old concrete. The effect of roughness on the bond behaviors and the failure mechanisms were intensively analyzed. The roughness of old concrete was first quantitatively characterized with a three-dimensional (3D) laser scanner, and the surface roughness index (SRI) was defined. Then, the old concrete specimens with different surface roughness indexes in the range of 1.0 to 1.5 were prepared and postcast with UHP-ECC. After that, the single-side shear test and splitting tensile test were conducted using the digital image correlation (DIC) technique. The results showed that the surface roughness of the old concrete significantly influenced the interfacial bonding properties. When the SRI was set as 1.3, the shear strength of the interface increased by 128.4% compared with the reference group, i.e., a plain concrete surface with SRI of 1.0. When the SRI was set as 1.4, the splitting strength of the interface increased by 221.24% compared with the reference group. However, the excessive roughness, e.g., when the SRI was set as 1.5, also led to the deterioration of interfacial properties due to the internal damage of the old concrete. Thus, it is concluded that the surface roughness index for the interfacial properties of UHP-ECC and concrete substrate is recommended in the range of 1.3-1.4.