Olanzapine is used as an atypical antipsychotic agent for the treatment of bipolar disorder and schizophrenia. Despite having only one rotatable bond, it exhibits a high degree of polymorphism in its solid state, giving rise to about 60 known distinct solid forms including four anhydrous polymorphs, 56 crystalline solvates, and an amorphous phase. Here, we focus on the four Z'=1 anhydrous forms. Among them, only three forms I, II, and IV have been characterized as single crystals, whereas form III was concomitantly found with form II. Form III was identified by comparing the PXRD patterns of form II and a mixed phase crystal of forms II and III. The presence of form III was also detected in a combined solid state NMR and CSP study, where a CSP generated structure in the Pbca space group was identified to possess the spectra characteristics of form III. We will refer to this structure as form III* in the following discussion.