The key challenges in finding efficient gas carriers for biomedical applications are related to the kinetics of gas adsorption/desorption of nontoxic materials over an appropriate time intervals, specifically targeting the affected area. Herein, we report the preparation of nanosized gadolinium (Gd-LTL) zeolite crystals with a diameter of 10−15 nm for oxygen delivery combined with magnetic resonance imaging (MRI) study. The partial ion-exchange of potassium (K) extra framework cations used for the synthesis of nanosized K-LTL zeolite with paramagnetic gadolinium (Gd, 1.53 wt%) changed considerably its adsorption capacity for O2. The nanosized Gd-LTL zeolite retained high colloidal stability, remarkable crystallinity and no leaching of Gd ions was observed. In addition, the in vitro toxicity study performed in human glioblastoma-derived cell lines revealed that nanosized K-LTL and Gd-LTL nanosized zeolites showed no toxicity even at high concentrations. This study provides promising guidelines regarding the design of smart biocompatible nanosized zeolites coupling gas-delivery and MRI proprieties.