Abstract Carrageenan is a seaweed polysaccharide widely used as a food gelling and thickening agent. The New Methylene Blue dye was tested as an analytical reagent for the quantification of carrageenan in commercial jelly samples. The main spectral peaks of the dye decrease with increasing carrageenan-to-dye molar ratio up to 1. Three analytical procedures were tested: (i) spectrophotometric measurement at 590 nm, (ii) smartphone-based color measurement within the RGB color model and (iii) spectrophotometric titration at 625 nm. All the procedures provide straight calibration lines. Spectrophotometry provides better linearity as compared to smartphone-based colorimetry. Range of determination is dependent on type and purity of carrageenan. The spectrophotometric titration procedure provides lower values of limit of determination due to longer optical path used. Typical food additives used in jelly deserts do not interfere significantly with the proposed analytical approach.