Although both Standby Letters of Credit and Contracts of Suretyship are employed to secure the performance of obligations, they possess fundamental differences in their theoretical foundations, legal structure, and practical function. The SBLC—based on principles such as independence from the underlying contract, the primary nature of the bank's obligation, its documentary character, being conditional upon the presentation of documents, and beneficiary protection—is considered an efficient instrument in international trade, enhancing transparency, speed, and certainty. In contrast, the Contract of Suretyship is rooted in civil law and is structured upon its accessory nature (dependency on the principal debt), the secondary and conditional obligation of the surety, and the establishment of the debtor's actual default. It is primarily utilized within traditional relationships and domestic legal systems. This study, employing an analytical-conceptual and comparative approach, elucidates these two legal instruments and analyzes their relative efficiency in responding to contemporary legal and economic requirements. The analysis indicates that SBLCs exhibit greater efficiency in international transactions, whereas suretyship remains effective in domestic relations; consequently, each is considered complementary to the other within its respective domain. Accordingly, the selection of an appropriate security mechanism must be contingent upon the nature of the obligation, the specific legal context, and the commercial exigencies of the transaction. Beyond its comparative analysis, this research offers practical implications for Iranian jurisprudence. It can be instrumental for courts and arbitral tribunals in clarifying the precise nature of a bank's undertaking in modern guarantee instruments and establishing its clear distinction from traditional civil law suretyship.