The described male has a light yellow carapace.
The back of the abdomen is pale yellowish with two dark transverse stripes near the rear.
In the described male Selenops petenajtoy, legs two, three and four are longer than the first. Its body measures 6.80 millimetres and its second leg 17.20 millimetres. The feet bear dense claw tufts and patches of specialised hairs on their terminal segments.
The specimens came from collections across a change of year. Two juveniles were found under bark near one entrance of Aktun Kan Cave, Guatemala, on 31 December 2007. On 1 January 2008, the name-bearing male was collected beneath bark on a large tree outside another entrance. The 2011 description included both collections. The name petenajtoy combines the locality name Peten with ajtoy, the Mopan Mayan word for spider, preserving both place and language.
An extinction-risk assessment has not been added yet.
Guatemala’s environment ministry reported an August 2022 reforestation activity with the ProPeten Foundation in the hills around Aktun Kan Cave. It stated that 100 participants planted 2000 trees there. This was regional work to increase tree cover around the locality where petenajtoy was found under bark.