Methods and devices for preventing urinary tract infections in catheterized patients are disclosed. The device may be a cylindrically shaped antimicrobial hydrogel having a central opening and a slit along its length so that it can be applied directly over a previously inserted urinary catheter. An optional locking mechanism disposed about the slit can be used to secure the antimicrobial device to a catheter shaft. The antimicrobial device has features that conform to tissues near both male and female urethral openings. The inner surface of the device is adapted to provide for releasable engagement with the shaft of the urinary catheter. In one embodiment, the device may be placed on a urinary catheter prior to inserting the catheter in a patient. Locking fittings to secure the antimicrobial device against the urethral opening may also be used. Antimicrobial agents can be incorporated within the hydrogel or coated onto its surface.