The device for interstitial laser thermotherapy of tumors comprises an hollow needle (3) with a point (3A) for perforating a tissue (T) to be treated, and a light guide (9) which can be inserted in said hollow needle. The hollow needle is associated with a membrane (17) which is inflatable to form a balloon (P) into which the distal end (9A) of said light guide (9) can project, the membrane being inflated to create a cavity in the tissue to be treated into which laser radiation is guided by said light guide.