Sn-based electrocatalysts are promising for the electrochemical CO 2 reduction reaction (CO2RR), but suffer from poor activity and selectivity. A hierarchical structure composed of ultrathin SnO x nanosheets anchored on the surface of the commercial multiwalled carbon nanotubes (MWCNTs) is synthesized by a simple hydrothermal process. The electrocatalytic performance can be further tuned by functionalization of the MWCNTs with COOH, NH 2 , and OH groups. Both SnO x @MWCNTs−COOH and SnO x @MWCNTs−NH 2 show excellent catalytic activity for CO 2 RR with nearly 100 % selectivity for C 1 products (formate and CO). SnO x @MWCNTs−COOH has favorable formate selectivity with a remarkably high faradaic efficiency (FE) of 77 % at −1.25 V versus standard hydrogen electrode (SHE) and a low overpotential of 246 mV. However, SnO x @MWCNTs−NH 2 manifests increased selectivity for CO with higher current density. Density functional theory calculations and experimental studies demonstrate that the interaction between Sn species and functional groups play an important role in the tuning of the catalytic activity and selectivity of these functionalized electrocatalysts. SnO x @MWCNTs−COOH and SnO x @MWCNTs−NH 2 both effectively inhibit the hydrogen evolution reaction and prove stable without any significant degradation over 20 h of continuous electrolysis at −1.25 V versus SHE.