PHD, with variations in capitalization and punctuation, is a three letter acronym that may refer to:
- Ph.D., the academic degree of Doctor of Philosophy
- Ph.D. (band), a 1980s British group
- PHD finger, a protein sequence
- PHD Mountain Software, an outdoor clothing and equipment company
- PhD Docbook renderer, an XML renderer
- PhD: Phantasy Degree, a Korean comic series
- PHD, an Italian pizza shop based in Hong Kong
- Phospholipid
- Piled Higher and Deeper, a web comic
- Parisada Hindu Dharma, an Indonesian reform organization
- Pisarenko harmonic decomposition, a method of frequency estimation
- Prolyl hydroxylase, an enzyme
- "PHD", a song by the Crystal Method from Tweekend
- Post hole digger