Terahertz (THz) communications are envisioned as a key technology to enable massive Internet-of-things (IoT) applications in the 6G era. In particular, THz communications intend to alleviate the capacity limitations of future wireless networks by exploiting the THz electromagnetic spectrum. However, despite the fact that the transmitted signals are highly directional in the THz frequencies, an adversary can intercept signals in line-of-sight transmissions. This chapter will provide a detailed review of physical layer attacks against THz communications and examine existing physical layer security (PLS) schemes to identify potential PLS countermeasures for protecting THz communications and the massive IoT applications relying on them in the 6G era. First, the fundamental trade-offs are presented associated with data eavesdropping from directional THz links. Later, potential attacks are detailed and analyzed, specifically targeting THz communications. The discussion proceeds with possible countermeasures to further increase the level of security in IoT services using directional THz radios. Finally, the chapter overviews other security-related challenges, such as the jamming of THz links.