The　use　of　terahertz　time-domain　spectroscopy　(THz-TDS)　for　the　nondestructive　testing　and　evaluation　(NDT&E)　of　materials　and　structural　systems　has　attracted　significant　attention　over　the　past　two　decades　due　to　its　superior　spatial　resolution　and　capabilities　of　detecting　and　characterizing　defects　and　structural　damage　in　non-conducting　materials.　In　this　study,　the　THz-TDS　system　is　used　to　detect,　localize　and　evaluate　hidden　multi-delamination　defects　(i.e.,　a　three-level　multi-delamination　system)　in　multilayered　GFRP　composite　laminates.　To　obtain　accurate　results,　a　wavelet　shrinkage　de-noising　algorithm　is　used　to　remove　the　noise　from　the　measured　time-of-flight　(TOF)　signals.　The　thickness　and　location　of　each　delamination　defect　in　the　z-direction　(i.e.,　through-the-thickness　direction)　are　calculated　from　the　de-noised　TOF　signals　considering　the　interaction　between　the　pulsed　THz　waves　and　the　different　interfaces　in　the　GFRP　composite　laminates.　A　comparison　between　the　actual　and　the　measured　thickness　values　of　the　delamination　defects　before　and　after　the　wavelet　shrinkage　denoising　process　indicates　that　the　latter　provides　better　results　with　less　than　3.712%　relative　error,　while　the　relative　error　of　the　non-de-noised　signals　reaches　16.388%.　Also,　the　power　and　absorbance　levels　of　the　THz　waves　at　every　interface　with　different　refractive　indices　in　the　GFRP　composite　laminates　are　evaluated　based　on　analytical　and　experimental　approaches.　The　present　study　provides　an　adequate　theoretical　analysis　that　could　help　NDT&E　specialists　to　estimate　the　maximum　thickness　of　GFRP　composite　materials　and/or　structures　with　different　interfaces　that　can　be　evaluated　by　the　THz-TDS.　Also,　the　accuracy　of　the　obtained　results　highlights　the　capabilities　of　the　THz-TDS　for　the　NDT&E　of　multilayered　GFRP　composite　laminates.　©　2023,　The　Author(s).