Self-discharge tuning of supercapacitors is critical in prolonging their energy retention, which would significantly benefit their applications in energy storage if the self-discharge process can be controlled. This report on self-discharge mechanisms of supercapacitors built with single-walled carbon nanotubes (SWNTs) demonstrates the effects of surface chemistry on self-discharge by interfering in the electrostatic interaction between electrolytic ions and the SWNT surface and also explores the tunability of the self-discharge process. Experimental results give decent correlation between the self-discharge performance and the concentration of functional groups attached to the electrode surface. The time the self-discharge process takes to reach half of the charged voltage is extended by ~5 times with functional group decreasing by ~13%. Tuning of the self-discharge rate is therefore feasible for the SWNT supercapacitors by simply trimming the surface chemistry of the nanotubes.