The optical absorption spectrum of the quasi-two-dimensional (2D) antiferromagnet RbtMnCI, is investigated in the region of the sAlg -+ 4Tzg(4D) transition (D group bands). It is found that short-range magnetic order strongly influences the absorption spectrum at temperatures T > T,.The spin-flop phase transition is observedin a magnetic field of 56 kOe. Investigation of theZeeman effect allows to conclude the spectrum in this region to consist of magnon and magnon-phonon sidebands of three different purely excitonic lines.