Copper electrodes are commonly employed in micro-resistance spot welding (MRSW), a dominant process used to join ultra-thin metallic sheets. During welding, some copper from the electrodes inevitably diffuses into the spot welds, changing the chemical compositions and properties of the resulting welded joints. In this study, 0.05-mm-thick Ti alloy metallic sheets were welded via MRSW under various combinations of welding parameters (ramping time, welding time, holding time, welding current, and electrode force). The effects of these welding parameters on electrode elemental diffusion were investigated via elemental analysis. Elemental composition of welded joints was measured via energy-dispersive spectrometry after tensile-shear tests. No copper was detected in the heat-affected zone or base material, but the amount of copper in the welding nuggets varied significantly with the welding parameters. Moreover, comparing copper element and hardness maps in weld nugget, the welding nugget hardness increased when more copper diffused into it.