Phlyctis sirindhorniae is a little-known species of corticolous (bark-dwelling), crustose lichen in the family Phlyctidaceae.

The type specimen was collected on the trunk of the tree Lophopetalum wallichii in a dry dipterocarp forest near Ramkhamhaeng University Chaloem Phrakiat Campus in Nong Bua Lamphu province, Thailand.

[2] Phlyctis sirindhorniae has a corticolous, crustose thallus, which is off-white to greyish-white and ranges up to 5 cm (2.0 in) in diameter and 80–130 μm thick.

[2] This lichen species is similar to Phlyctis agelaea due to their muriform ascospores and presence of norstictic acid.

[2] Phlyctis sirindhorniae is found on the trunks of Lophopetalum wallichii trees in dry dipterocarp forests in northeastern Thailand, specifically near Ramkhamhaeng University Chaloem Phrakiat Campus in Nong Bua Lum Phu.