Thermoelectric (TE) materials convert waste heat into electricity. Hence, they are foreseen as a possible source of green energy. Compared to traditional TE materials like bismuth telluride or other metal oxides, polymeric materials are much cheaper and easier to be fabricated into devices. Biodegradable materials are of special interest as they abide to ‘fit and forget’ system where there is no need of disposal concern. Cellulose is one of the most common biodegradable materials, which is also conducive to the formation of aerogels. To exhibit high TE properties, materials should have high electrical conductivity, high Seebeck coefficient (S) and low thermal conductivity. In this work, we prepared composites based on cellulose and carbon nanotubes (CNTs), a highly anisotropic electrically and thermal conductive thermoelectric filler, and studied the effect of types (single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs)), amount (2-10 wt%) and morphology (films and aerogels) on the TE properties. Composite films based on SWCNTs showed significantly higher electrical conductivities (5 S/cm at 10 wt%) and S values (47.2 μV/K at 10 wt%) compared to those based on MWCNTs (0.9 S/cm and 11 μV/K, respectively). Lyophilization decreased the electrical conductivity for both types by one order of magnitude, but did not affect the S value of MWCNT based nanocomposites. For SWCNT containing aerogels, higher S values than for films were measured at 3 and 4 wt% CNT contents but significantly lower values at higher loadings. CNT addition increased the thermal conductivity from 0.06 to 0.12 W/(m∙K) in the films, whereas the lyophilization significantly reduced it towards values between 0.01 and 0.09 W/(m∙K) for the aerogels. The maximum Seebeck coefficient, power factor, and ZT observed in this study are 49 μV/K for aerogels with 3 wt% SWCNTs, 1.1 μW/(m∙K2) for composite films with 10 wt% SWCNTs, and 7.4×10−4 for films with 8 wt% SWCNTs, respectively.