Placidus Braun, OSB (1756–1829), Bavarian Benedictine priest, historian and archivist
Lactantius Placidus (350–400 AD) was the presumed author of a commentary on Statius's poem Thebaid.
The Lake Placid Funnel wolf spider (Sosippus placidus) is a species of spider in the Lycosidae family.