The presence of emulsified water in heavy crude oils invariably causes processing problems and financial costs associated with an increased energy input required to handle highly viscous fluids. Thus, physical or chemical separation methods are usually applied to dehydrate crude oils before processing. Dehydration chemicals or demulsifiers are among the most widely used chemical strategies to rid heavy crude oil from water. In this work, we explore the use of amidated cellulose nanofibrils as dehydrating agents for a water-in-heavy-crude-oil emulsion. Using microcrystalline cellulose (MCC) as raw material, we extracted oxidized cellulose nanofibrils (1.25 mmol COOH/g cellulose) via ultrasound-assisted TEMPO oxidation. TEMPO-oxidized cellulose nanofibrils (TOCNs) were rendered hydrophobic through the coupling with dodecyl- and octadecylamine through one-pot amidation reactions. The introduction of C12 and C18 alkyl moieties on the cellulose surface was enough to promote the dispersion of the materials in toluene and increase the water contact angle (CA) from 17° for TOCN to 61° and 67° for TOCN-AMDC12 and TOCN-AMDC18, respectively. HLB values of 9.6 and 9.2 for TOCN-AMDC12 and TOCN-AMDC18 are characteristic of dehydrating agents for w/o emulsions. Thus, the materials were tested as demulsifying agents for the disruption of a natural water-in-heavy-crude-oil emulsion with a water content of 55%. Conventional tests (BSW, bottle test) showed the ability of TOCN-AMDC12 and TOCN-AMDC18 to effectively dehydrate the emulsions, with a water recovery up to 74%, when used at 1500 ppm in toluene as the carrier fluid. Optical microscopy and rheological analysis were performed before and after the nanofluids' application to determine the viscosity profile of the emulsion and to observe the types and sizes of water droplets in the oil phase after dehydration.