A new, not yet described double salt K2Mn3(SO4)4 · 5H2O has been found. Single crystals were prepared using the diffusion-reaction method. It crystallizes in the P21/a space group with parameters a = 9.969(2) Å, b = 9.750(2) Å, c = 19.168(4) Å, β = 103.96° and Z = 4. The new double salt has been characterized thermogravimetrically, by IR spectra and by X-ray studies. Data for the dielectric constant of K2Mn3(SO4)4 · 5H2O as function of the temperature have been obtained.