A femtosecond laser is a laser which emits optical pulses with a duration well below 1 ps (→ ultrashort pulses), i.e., in the domain of femtoseconds (1 fs = 10−15 s). It thus also belongs to the category of ultrafast lasers or ultrashort pulse lasers (which also include picosecond lasers).

The generation of such short (sub-picosecond) light pulses is nearly always achieved with the technique of passive mode locking. That leads to pulse trains with high pulse repetition rates in the megahertz or gigahertz region. This combined with the limited average output power leads to relatively small pulse energies (often in the nanojoule region). Far higher pulse energies (at lower repetition rates) – often higher by many orders of magnitude – are possible by using some kind of optical amplifiers system (→ ultrafast amplifiers) in addition to a femtosecond laser.