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Phenolic resins are an attractive family of thermoset materials. However, there are some concerns related to the high toxic nature of formaldehyde and phenol, i.e., the main building blocks for phenolic resins. Herein, a sustainable alternative method is proposed to synthesize fully biobased and nonharmful resoles based on the condensation of resorcinol (R) with 4-four different aldehydes under alkaline conditions. Two aldehyde-based building blocks including 5-methylfurfural (MFu) and cinnamaldehyde (C) obtained from biobased renewable and abundant resources were successfully formulated with resorcinol, providing more sustainable and environmental-friendly alternatives to current commercially available and hazardous phenol-formaldehyde resins. Two molar ratios of basic catalyst (HO–)/(R) were considered to study their effect on the prepolymerization reaction and on the thermo-mechanical properties of the final materials. The thermal decomposition behavior of the cured resoles was investigated by thermogravimetric analysis (TGA), showing high degradation temperatures and high char yields (up to 60%). Furthermore, the resoles also exhibited high chemical resistance with insoluble fractions (IFs) up to 80%. Finally, to study the mechanical properties of the obtained resoles, the fully cross-linked resoles were consolidated using the spark plasma sintering (SPS) technique. The microstructures of the sintered resoles were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Compression tests were also conducted to evaluate the mechanical resistance of cured resoles with compression moduli and strength values ranging up to 1.25 GPa and 74 MPa, respectively. Pyrolysis–combustion flow calorimetry analysis (PCFC) revealed that MFu has a very low flammability behavior, demonstrated with a peak of heat release (pHRR) of around 16 W/g and the total heat release (THR) of 2.5 kJ/g. Overall, it was demonstrated that MFu as a nontoxic, biobased, and inexpensive aldehyde can be advantageously used in the preparation of more sustainable phenolic resins.