This study investigates the positive effects of an appropriability regime and perceived similarity to green co-innovation performance when considering green open innovation activities as the mediator. It proposes a novel construct, i.e., green open innovation activities, and employs structural equation modeling to test its hypotheses. 190 valid questionnaires were collected from executives in Taiwanese manufacturing companies. Manufacturing activities are regarded as a major source of pollution. Consequently, given the broad concern for the environment among governments and consumers, adopting green practices has become critical for manufacturing companies. All the proposed hypotheses were supported by the analysis results. An appropriability regime is positively associated with green open innovation activities and green co-innovation performance. Perceived similarity is positively associated with green open innovation activities and green co-innovation performance. Moreover, green open innovation activities are positively associated with green co-innovation performance. A major finding is that if a company introduces one, the longer the duration of a green project is, the stronger the green open innovation activities and green co-innovation performance are. This study aimed to determine the simultaneous effects of both factors, i.e., appropriability regime, and perceived similarity on green open innovation activities and green co-innovation performance. The contribution of this study highlights the simultaneous importance of appropriability regimes and perceived similarity to determine a company’s green practices. While companies have tended to increase their green co-innovation performance, they need to improve their appropriability regime, perceived similarity, and green open innovation activities.