Selective detection and effective therapy of brain cancer, specifically, the very aggressive glioblastoma multiforme (GBM), remains one of the paramount challenges in clinical settings. While radiotherapy combined surgery is proposed as the main treatment course, it has several drawbacks such as complexity of the operation and common development of recurrent tumors in this course of patient care. Unique opportunities presented by photodynamic therapy (PDT) offer promising, effective, and precise therapy against GBM cells along with simultaneous imaging opportunities. However, activatable, theranostic molecular systems in PDT modality for GBM remained scarce. Specifically, even though elevated fi-galactosidase (fi-gal) activity in glioblastoma cells is well-documented, targeted, activatable therapeutic PDT agents have not been realized. Herein, we report a fi-galactosidase (fi-gal) activatable phototheranostic agent based on an iodinated resorufin core (RB-1) which was realized in only three steps with commercial reagents in 29% overall yield. RB-1 showed very high singlet oxygen (1O2) quantum yield (54%) accompanied by a remarkable turn-on response in fluorescence upon enzymatic activation. RB-1 was tested in different cell lines and revealed selective photocytotoxicity in U-87MG glioblastoma cells. Additionally, thanks to almost 7% fluorescence quantum yield (phi F) despite extremely high 1O2 generation yield, RB-1 was also demonstrated as a successful agent for fluorescence imaging of U-87MG cells. Due to significantly lower (fi-gal) activity in healthy cells (NIH/3T3), RB-1 stayed in a passive state and showed minimal photo and dark toxicity. RB-1 marks the first example of a fi- gal activatable phototheranostic agent toward effective treatment of glioblastoma.