This paper deals with adsorption of a cationic dye – methylene blue - on the activated biocarbon obtained via chemical activation of horsetail herb post-extraction residue. The effects of initial dye concentration, temperature, solution pH, as well as the phase contact time on the adsorption effectiveness have been tested. The adsorbent prepared was characterized based on the results of elementary analysis, surface area measurements, thermal analysis as well as determination of the acid-base nature of its surface. It has been shown that micro/mesoporous horsetail herb-based activated biocarbon exhibits a significantly higher ability to methylene blue uptake than the commercial activated carbon prepared from peat. It has been also found that the adsorption capacity increases with the increasing initial dye concentration in the solution and the phase contact time. The maximum adsorption toward methylene blue was found at pH=12 and at temperature of 40 C. A better fit to the experimental data was obtained with a Langmuir isotherm than Freundlich one, whereas better fit of the kinetic data was achieved using the pseudo-second order model.