The paper presents the process of cellulose extraction from rice straw using water–alkaline solution treatment and the subsequent process of carboxymethylation of the obtained product when activated by microwave radiation. After mercerization of rice straw, the obtained product contained 89.2% cellulose and 6.7% lignin. The X-ray diffraction pattern of the obtained lignocellulose shows three diffraction peaks in the region typical for the polymorphic modification of cellulose Iβ (2θ = 15.50(78), 21.70(145), 34.70(52)). The degree of crystallinity was 65%. The product was heat-stable up to 247 °C. The synthesis of carboxymethylcellulose (CMC) based on the obtained product included successive processes of thermostating in alcohol–alkali solution and cellulose esterification reaction using monochloroacetic acid. To activate the carboxymethylation process, microwave radiation was used (350 W for 90 s), which made it possible to reduce the reaction time by more than 100 times. Functional group analysis of the carboxylated lignocellulose from rice straw was carried out using an FTIR spectrometer. In the IR spectra, a band with a maximum of 1742 cm⁻¹ was recorded, corresponding to stretching vibrations of >C(O)OH groups. The degree of polymerization was recorded by mass spectrometry.