The primary treatment for glioblastoma (GBM) is removing the tumor mass as defined by magnetic resonance imaging (MRI). However, MRI has limited diagnostic and predictive value. Tumor-associated macrophages (TAMs) are abundant in GBM microenvironment (TME) and are found in peripheral blood (PB). FKBP51 expression, with its canonical and spliced isoforms, is constitutive in immune cells and aberrant in GBM. Spliced FKBP51s supports M2-polarization. To find an immunological signature that combined with MRI could advance in diagnosis, we immunophenotyped the macrophages of TME and PB from 37 GBM patients using FKBP51s and classical M1-M2 markers. We also determined the tumor levels of FKBP51s, PD-L1, and HLA-DR. Tumors expressing FKBP51s showed an increase in various M2 phenotypes and Tregs in PB, indicating immunosuppression. Tumors expressing FKBP51s also activated STAT3 and were associated with reduced survival. Correlative studies with MRI and tumor/macrophages co-cultures allowed to interpret TAMs. Tumor volume correlated with M1 infiltration of TME. Co-cultures with spheroids produced M1 polarization, suggesting that M1 macrophages may infiltrate alongside cancer stem-cells. Co-cultures of adherent cells developed the M2 phenotype CD163/FKBP51s expressing pSTAT6, a transcription factor enabling migration and invasion. In patients with recurrences, increased counts of CD163/FKBP51s monocyte/macrophages in PB correlated with callosal infiltration and was accompanied by a concomitant decrease in TME-infiltrating M1 macrophages. PB PD-L1/FKBP51s connoted necrotic tumors. In conclusion, FKBP51s identifies a GBM subtype that significantly impairs the immune system. Moreover, FKBP51s marks PB macrophages associated with MRI features of glioma malignancy that can aid in patient monitoring.