This paper presents a Hardware-assisted security primitive that integrates Physically Unclonable Functions (PUF) and IOTA Tangle for device authentication in the Internet-of-Medical-Things (IoMT). The increasing market and scope for the IoMT is due to its potential in enhancing and improving the efficiency of health services across the globe. As the applicability of IoMT is increasing, various security vulnerabilities are surfacing and hindering its adoption. Device and data security are pivotal for Healthcare Cyber-Physical Systems (H-CPS) since a vulnerable working ecosystem in healthcare to various security attacks could risk the patient’s lives. To ensure the authenticity of IoMT, the proposed security scheme uses Masked Authentication Messaging (MAM), which is the second level communication protocol for secure data storage, retrieval and sharing in IOTA Tangle. MAM works in three modes: Public, Private and Restricted. The proposed security primitive has been developed in Restricted mode for ensuring the utmost security by storing the PUF key of the IoMT in Tangle using MAM. PUFs are one of the most widely adopted hardware security primitives which work based on nanotechnology to build a secure fingerprint that guarantees the integrity of consumer electronic devices. For validating PUFchain 3.0, a strong arbiter PUF module, which supports higher number of Challenge Response Pairs (CRP), has been configured on two FPGA boards on both the IoMT and the edge server sides for validation. The proposed security scheme has taken less than 1 min to upload the transaction onto Tangle through MAM and less than 2 s to retrieve the data, which substantiates its robustness and potential for sustainable and secure Smart Healthcare.