The control of the geometry of carbon nanotubes has a high potential for electro-optical technology developments. We report here the use of a new spectroscopic signature for the detection and characterization of the pressure induced radial buckling of single walled carbon nanotubes using Raman spectroscopy combined with sapphire anvil cell pressure systems. We follow the appearance of a defect-free Raman D-band contribution in bundled samples which is assigned to the pressure-induced radial buckling, as already shown on radially collapsed nanotubes at ambient pressure. On pressure release, this contribution to the D-band disappears, confirming the reversibility of the process. We applied this approach to the study of isolated tubes and followed the collapse transition of a 1.7 nm diameter tube, most likely identified as a (16,8) chirality, starting at 1 GPa and ending at 2 GPa. Our study further illustrates the potential of utilizing the D-band as a new spectroscopic probe to explore geometrical changes in carbon nanotubes.