Smart contracts are self-executing programs on a blockchain to ensure immutable and transparent agreements without the involvement of intermediaries. Despite the growing popularity of smart contracts for many blockchain platforms like Ethereum, smart contract developers cannot prevent copying their smart contracts from competitors due to the absence of technical means available. However, applying existing software watermarking techniques is challenging because of the unique properties of smart contracts, such as a code size constraint, non-free execution cost, and no support for dynamic allocation under a virtual machine environment. This paper introduces a novel software watermarking scheme, dubbed SmartMark, aiming to protect the piracy of smart contracts. SmartMark builds the control flow graph of a target contract runtime bytecode and locates a series of bytes randomly selected from a collection of opcodes to represent a watermark. We implement a full-fledged prototype for Ethereum, applying SmartMark to 27,824 unique smart contract bytecodes. Our empirical results demonstrate that SmartMark can effectively embed a watermark into smart contracts and verify its presence, meeting the requirements of credibility and imperceptibility while incurring a slight performance degradation. Furthermore, our security analysis shows that SmartMark is resilient against foreseeable watermarking corruption attacks; e.g., a large number of dummy opcodes are needed to disable a watermark effectively, resulting in producing illegitimate smart contract clones that are not economical.