In the interfirm co-innovation process, knowledge search spanning organizational boundaries is believed to be essential to innovation but is often technologically, geographically, and socially bounded. Given that spanning any type of boundary may lead to both decreased learning and increased creativity, its influence on co-innovation success remains unclear. Using a sample of 18,575 US utility patents co-invented by 6,595 dyads of firms during 1993–2002, the net effect of interfirm technological distance (which depicts cognitive boundary-spanning search) on the quality of a co-invented patent is found to depend largely on other types of boundary-spanning searches, such as cross-regional and cross-network partnerships. The net effect is negative when co-inventing partners are within the same region and network, whereas the net effect becomes positive when partners originate from distinct regions and networks. The findings advance the understanding of boundary-spanning searches of various types and their complicated interactions, suggesting that boundary spanning is crucial to co-innovation success.