A blockchain serves a crucial role in enhancing trust and security in online transactions by providing a decentralized, immutable ledger. Each transaction is cryptographically secured and verified by a network of nodes, making it resistant to tampering and fraud. This transparency allows parties to independently verify transactions, fostering trust among users. Furthermore, the use of smart contracts automates and enforces agreements without intermediary involvement, thus reducing risks and increasing efficiency in processes like financial transactions or supply chain management.