Rappiidic acid, a new o-orsellinic acid derivative, was isolated from the lichen Cladonia rappii. Its capability to scavenge reactive oxygen species (ROS) and reactive nitrogen species (RNS) was investigated and compared with resveratrol and (+)-usnic acid. Usnic acid at 100 μM was the most efficient ROS scavenger, exhibiting activity 3-fold higher than that of resveratrol. At the same concentration, rappidic acid scavenged 23.1% of ROS formed, demonstrating that this compound is twice as active as resveratrol. Both compounds were shown to be poor RNS scavengers.