This paper reports on a new experimental study for the behaviour of reciprocally connected and supported Fibre Reinforced Polymer (FRP) hollow square profiles axially loaded under several boundary conditions. The study aims to determine the ultimate load of the assembly and failure mechanism of mutually connected units. For the tests, FRP reciprocal frames units (RF) of 100 × 100 × 6.4 mm thick square hollow sections were designed, fabricated and assembled using mechanical fasteners. A bespoke steel test rig allowed for varied support boundary conditions. The observed failure modes were dominated by web buckling, bearing failure around the bolted areas and localised failure. The 100 × 100 mm RF unit achieved the highest load capacity of 16.4 kN and frame stiffness of 1.7 kN/mm, under the pin-pin-roller support boundary conditions. This paper presents the experimental procedure, results and observations.