This article identifies barriers that small and medium-sized enterprises (SMEs) encounter when they openly innovate, according to the open innovation (OI) mode used (inbound, outbound, coupled). A qualitative analysis—involving seven case studies of SMEs active in digital (high-tech) or social economy (low-tech) sectors—reveals that they face more internal than external OI barriers. Overall, the nature of the barriers does not vary across OI modes, but their intensity does. With regard to external barriers, the results reveal a “tribe syndrome,” such that SMEs resist opening up to other firms that do not share the same values.