An implantable vascular access port assembly that has a septum, a reservoir, and a catheter connector assembly for use in connecting a catheter with the vascular access port. The catheter connector assembly has an O-ring, an O-ring seal, a locking ring, and a catheter connection plug, which components are located in secured adjacent relationship to one another, respectively, in the vascular access port. The catheter connector assembly securely locks the catheter inside of the vascular access port without compromising the lumen cross-sectional area and can withstand high pressures.