The fluid-structure interactions (FSIs) are challenging when the structures are flexible due to their intricate coupling nature. Numerical simulations of these interactions are even more challenging. Only in the recent days, there has been progress in the FSI solvers for such interactions. The solvers are validated at low Reynolds numbers against the numerical results. We consider an experimental investigation of flow behind square cylinder with flexible splitter filament at low Reynolds number which can be treated as a test case for numerical solver validation. We experimentally investigate the kinematics of a flexible splitter filament attached behind a square cylinder at \(Re_D =498\) in a soap film tunnel. The length of the flexible filament is L, and the side of the square cylinder is D. We varied the length of the filament from \(L^*= 1\) to \(L^*= 5\) , where \(L^*= L/D\) . Flow visualization using thin film interferometry and filament visualization were performed. The Fast Fourier Transform was performed on the time evolution of the tip locus to obtain the frequency spectrum. Detailed study on the frequency spectrum and the flow visualizations reveals that the filament length has a predominant effect on it’s kinematic behavior. For smaller length ( \(L^*= 1\) ), the filament tip oscillates with small amplitudes near the Mean Splitter Line (MSL). For \(L^*= 2\) , the filament reorients itself to one side of the MSL breaking the symmetry. From \(L^*= 3\) onward, the filament oscillates periodically and nearly symmetrically about the MSL. From \(L^*= 3\) to \(L^*= 5\) , the tip amplitude increases almost linearly with the length, whereas the tip oscillation frequency decreases. The vortex shedding frequency which is the same as the tip oscillation frequency decreases with increase in filament length ( \(L^*= 3\) to \(L^*= 5\) ).