Security Requirements Engineering (SRE) deals with the specification of security requirements for the system-to-be starting with the analysis of security issues as soon as in the early requirements phase. STS-ml is an actor- and goal- oriented requirements modelling language for Socio-Technical Systems (STSs), which represents the security needs the stakeholders express as constraints over the interactions between actors. In this paper, we present STS-Tool, the secu- rity requirements engineering tool that supports STS-ml. STS-Tool allows for modelling a socio-technical system at a high level of abstraction, expressing con- straints (security needs) over the interactions between the actors in the STS, and deriving security requirements in terms of social commitments (promises with contractual validity). It offers multi-view modelling, allowing designers to focus on a different perspective at a time, while promoting modularity.