Biological clock may refer to:

• Age and female fertility, decrease of female fertility with advancing maternal age
• Ageing, biological program that limits the lifespan of an individual
• Biological rhythms
• Internal clock, the timeframe sensed by the individual's body which is largely affected by the light-dark cycle
• Circadian clock, a molecular mechanism that results in a circadian rhythm in a living organism
• Circadian rhythm, biological process that displays an oscillation about 24 hours, such as the human sleep-wake cycle (the "body clock")
• Epigenetic clock, a set of DNA sites whose methylation levels can be used to measure aging throughout the body
• Molecular clock, a technique that uses the mutation rate of a biomolecule to deduce the time in prehistory when two life forms diverged
• Vernalisation, the induction of flowering by prolonged exposure to low temperatures, as during the winter in a temperate climate
• Menstrual cycle, the regular natural change that occurs in the female reproductive system that makes pregnancy possible