We demonstrate that iodine-doping into single-walled carbon nanotubes (SWCNTs) can be effectively done using an electrochemical method. It is shown by in situ Raman measurements that the iodine-doping level can be easily and finely controlled because de-doping is also possible by changing the polarity. In situ synchrotron XRD measurements reveal that iodine molecules are mainly inserted into the hollow core of SWCNTs. The dispersion state of the iodine-doped SWCNTs in water as a function of temperature is also investigated. It is shown that the iodine-doped SWCNTs can be homogeneously dispersed in water at low temperature (ca. <15 °C).