<p>This study investigated the effect of varying end conditions on vortex-induced vibration (VIV) of a vertically mounted circular cylinder at the subcritical to critical Reynolds number (<i>Re</i>) range (3.14×10⁴–2.51×10⁵). Three endplate configurations were tested: upper and lower endplates, upper endplates only, and no endplates. Results showed that end conditions have a minor impact on peak amplitude but significantly alter the vibration branches. Notably, the no endplates configuration exhibited two amplitude peaks, with endplate effects becoming more significant above a <i>Re</i> of 1.0×10⁵. The frequency ratio remained within the lock-in region for all high <i>Re</i> tests, and with no endplates, the ratio stayed closer to unity. The effect of end conditions on hydrodynamic forces mirrored the amplitude response: Peak drag and lift forces were similar across all configurations. However, with no endplates, drag force remained substantially higher than the other conditions within a reduced velocity range of 6–8, and the lift force showed two peak values. Phase difference analysis revealed two phase jumps for both the only upper and no endplates configurations, corresponding to an expanded upper branch. These observed differences in amplitude and hydrodynamic forces were interpreted based on instantaneous pressure distributions.</p>