A portable deadbolt lock security device is configured to prevent the handle of the deadbolt lock from being turned. The device includes a flexible strap having a first end, a second end, and a medial portion. An opening is located in the first end of the strap. A fastener is provided for mutually engaging the second end of the strap with the medial portion. In use, the handle of the deadbolt lock is turned to its locked position and inserted into the opening in the first end of the strap. Subsequently, the second end of the strap is encircled about the stem of a doorknob, pulled taut, and brought into mutual engagement with the medial portion of the strap.