Multiwalled carbon nanotubes (MWNTs) were functionalized with four different functional groups and used as nanofillers to reinforce biodegradable polyurethanes (PU), in an effort to explore their effect on the thermal and mechanical properties of the obtained nanocomposites. The PU were synthesized via one step process, by the reaction between a αω‐telechelic polyester diol (obtained by biocatalysis from ε‐caprolactone and diethylene glycol), and hexamethylene diisocyanate. The MWNTs were synthesized via a chemical vapor deposition process using ethanol as a carbon source. Functionalization with (i) hydroxyl and carboxyl groups OH‐COOH; (ii) aliphatic amine: ethylene diamine; (iii) aromatic amine: 4‐amine benzoic acid (4AB); or (iv) aromatic amine and carboxyl acid: 4‐amine benzoic acid, hidroxyl, and carboxyl groups (4AB‐OH‐COOH) were achieved. Nanocomposites were obtained by adding functionalized CNTs during the polymerization reaction between αω‐telechelic diol and the isocyanate. Three different percentages of functionalized CNTs were tried: (a) 0.25; (b) 0.5; or (c) 1%wt. The structural, morphological, and mechanical features of MWNTs/PU nanocomposites were studied by FT‐IR, differential scanning calorimetry, field emission scanning electron microscopy, TGA, wide angle X‐ray diffraction, solid‐state NMR, and mechanical properties. A better interfacial adhesion was observed for nanocomposites containing 0.5% wt of filler, compared to the neat PU. POLYM. COMPOS., 39:E697–E709, 2018. © 2016 Society of Plastics Engineers