Under the terms of the Creative Commons Attribution (CC BY) license to their work. ; We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality. ; The authors acknowledge the financial support from the FP7 project MERGING (Grant No. 309150), the Spanish MICINN projects nanoTHERM (Grant No. CSD2010–0044), TAPHOR (MAT2012–31392), and the Academy of Finland (Grant No. 252598). E.C.-A. gratefully acknowledges a Becas Chile 2010 CONICYT fellowship from Chilean government. ; Peer Reviewed