Injectable oxygen delivery is an emerging technology that presents an opportunity for improved patient care in a number of medical disciplines. Here, we report on the fabrication and characterization of novel protein-encapsulated oxygen microbubbles (OMBs) designed for intravenous injection. The nanothick albumin encapsulation provided OMBs small enough for transcapillary passage: 99% of the microbubbles were less than 3-μm diameter and less than 1% of the oxygen was encapsulated in microbubbles greater than 8-μm diameter. The protein OMBs were remarkably stable, losing less than 40% of the encapsulated gas over 12 days. Upon injection into an oxygen-depleted saline solution, the protein OMBs rapidly equilibrated by releasing their oxygen core. These results indicate that protein microbubbles may serve as a suitable platform for direct injection of bioactive and therapeutic gases.