A new bio-sourced adhesive system is presented based on urea crosslinked glyoxalated lignin and dialdehyde starch obtained by hydrogen peroxide oxidation of starch. The molecular species formed and the reactions mechanism involved were identified by FTIR, 13C NMR, and MALDI-ToF mass spectrometry. Urea appears to react with both, glyoxalated lignin and dialdehyde starch. The adhesives based on this reaction were tested by means of laboratory particleboards, differential scanning calorimetry (DSC), and thermomechanical analysis (TMA). The results obtained for internal bond (IB) strength with this adhesive system are acceptable for producing particleboard according to the relevant European standards. Additionally, chemical crosslinking by using 5% on resin solids of partially bio-sourced glycerol glycidyl ether (GDE) improves the bonding strength of the resins so formed; the main crosslinking sites were also determined.