The purpose of this research is to empirically reveal the effect of external technology R&D cooperation network diversity (ETRDCND) on the greenhouse gas (GHG) emission reduction and energy saving of small and medium-sized enterprises (SMEs). Besides this, this study aims at analyzing the roles of production time reduction and absorptive capacity in the relationship between SMEs’ ETRDCND and their GHG emission reduction and energy saving. GHG emission and energy usage have been playing a crucial role in aggravating global warming. Global warming results in big problems such as worldwide unusual weather and health disorders. SMEs play a substantial role in the industrial growth of the global economy, which increases GHG emission and energy consumption. By performing the ordinary least squares regression with the data of 3300 South Korean SMEs, this research reveals four points. First, ETRDCND positively influences SMEs’ GHG emission reduction and energy saving. Second, production time reduction perfectly mediates the relationship between SMEs’ ETRDCND and their GHG emission reduction and energy saving. Third, the mediating role of production time reduction in this relationship is moderated by SMEs’ absorptive capacity. Fourth, ETRDCND significantly influences SMEs’ GHG emission reduction and their energy saving only if SMEs possess their own absorptive capacity.