The present invention provides transseptal puncture devices configured to access structures on the left side of the heart from the right side of the heart without requiring open-heart surgery. The devices have adjustable stiffness to enter the vasculature in a flexible, atraumatic fashion, then become rigid once in place to provide a stable platform for penetration of the fossa ovalis. The devices are further configured to controllably and stably extend a needle to puncture the FO. The devices include an indwelling blunt stylus that can extend perpendicularly from the device to increase the accuracy of placement near the fossa ovalis.