•Vertical post-tensioned (PT) connection for modular steel buildings is presented.•The connection comprises of a post-tensioned threaded rod and a steel box.•The post-tensioned threaded rod is installed inside the hollow steel columns.•The steel box is placed between two vertically consecutive columns.•The PT connection eliminates on-site welding for vertical intermodular connection.•Specimens with PT and welded connection were tested under quasi-static cyclic load.•The behavior of specimens with PT and with welded connection are compared. Modular construction is an off-site construction technique. In this method, structural volumetric modular components are produced in a factory and assembled on-site to form a larger, permanent building. Typical vertical connections of modular steel buildings (MSBs) are provided by on-site welding. Welding may interfere with the finishing of the modules and also when several modules are placed together at a given floor level complete access for welding is compromised. As an alternative to on-site welding, the present paper proposes a new vertical post-tensioned (PT) connection for MSBs. This connection is comprised of a post-tensioned threaded rod installed inside hollow structural sections (HSS) columns and a steel box placed between two modules. In order to evaluate the general and the seismic performance of the proposed connection, eight quasi-static cyclic loading tests were performed in T-shaped subassemblies. A combination of three different steel boxes and three initial post-tensioning loads levels were considered. Additionally, two quasi-static cyclic loading tests were performed using standard welded connections. No local buckling was observed in any of the specimens and no welding fractures occurred up to 3% drift demand. Results indicated that in comparison to the welded connection the proposed PT connection have similar lateral stiffness and strain distribution, and a higher cumulative energy dissipation capability. Therefore, the proposed connection has the potential to eliminate on-site welding in the assembly of the modules while providing the lateral resistance required.