Successful new product development (NPD) is considered key to the survival and growth of a company. In today's business world, there are many firms riding on the wave of globalization to rapidly introduce new products through business networking or corporate collaboration. This is noticeable in the technology industries wherein firms generate, share, interpret and use new knowledge jointly with their different business partners, and in particular during inter‐firm NPD. However, there are also decision‐makers in these firms concerned about knowledge sharing with others which would invite opportunism, i.e. knowledge being copied or leaked to other parties or competitors. As a result, opportunistic problems around knowledge sharing suffocate innovative ideas and thus negatively impact NPD. A legal contract is a well‐known governance tool to mitigate opportunism. But literature also indicates that merely relying on a contract is not the complete solution for protecting firms from opportunism in today's business environments which is getting more complex. This study examines the impact of knowledge sharing in inter‐firm NPD in relation to inter‐firm contractual, commitment and trust relationships facing with the threat of opportunism. Through analysing empirical data from 312 valid responses by structural equation modelling, our findings not only reconfirm that a legal contract has the ability to suppress opportunism in inter‐firm NPD activities, it also indicates that a contract can motivate knowledge sharing if the inter‐firm relationship is mediated by a strong trust and commitment among firms. A further surprising finding is that, with the presence of a formally stipulated contract and a well‐established foundation of trust and commitment, the actions of more knowledge sharing among committed partnering firms can reduce the incidence of opportunism in NPD. Further investigation suggests strong trust and commitment between contractual related business partners encourage their willingness to share critical knowledge which in return is a strong signal to these partners they trust each other more. This manifestation of greater trust further induces these partners to commit more to the relationship and enhances the willingness to share more knowledge. This positive interlocking effect in the inter‐firm relationship and knowledge sharing act together would further reduce opportunism. This controversial finding contradicts many past research findings and presents a new understanding of successful inter‐term knowledge sharing with strong managerial implications to encourage knowledge sharing among business partners.