Ultra-high performance fiber-reinforced concrete (UHPFRC)-filled steel tube (UHPFRC-FST) slender columns have not been studied to date, although ample research exists on this system as short columns. Furthermore, current code provisions (e.g. CAN/CSA S16) were developed for normal strength concrete and are thus potentially inadequate for UHPFRC-FSTs. In this study, a robust three-dimensional nonlinear finite element model was developed and validated using LS-DYNA software to simulate behaviour of slender UHPFRC-FSTs of circular cross-section under concentric axial compressive loads. An extensive parametric study was then performed, varying slenderness ratio (kL/r) based on column length (L) where r is the radius of gyration, diameter-to-thickness ratio (D/t) of the steel tube, effective length factor (k), and tube’s yield strength (Fy). The study showed that axial load-carrying capacity reduced by 78% when slenderness ratio increased from 12 to 157. The column transitions from material failure by diagonal shear plane in concrete, to global buckling at a slenderness ratio of about 80. Axial load-carrying capacity also reduces rapidly with D/t until a ratio of about 45 and then reduces at a much lower rate thereafter. CAN/CSA S16 code provisions grossly underestimate the load-carrying capacity of slender UHPFRC-FSTs by 13 to 46% as slenderness ratio increased from 12 to 157. The underestimation also increased dramatically with diameter-to-thickness ratio, from 8% atD/t = 10 to 40% atD/t = 80, and with effective length factor k, by 15% at k=0.5 to 48% at k=2. 0. A modification to CAN/CSA S16:19 equation is proposed based on multiple regression analysis of the results of the parametric study.