A blood seal having a spring-biased septum is provided for preventing the leakage of blood during the placement and use of vascular catheters and similar devices. The structure of the blood seal includes a housing with an internal channel. A spring-biased septum is placed securely within the channel such that it substantially blocks blood flow through the channel. The spring-biased septum includes an elastic plug having a pre-slit or pre-molded hole and a biasing element disposed about the elastic plug. A needle or other object may be withdrawn through the slit or hole, after which the biasing element forces the hole closed, preventing blood leakage.