Padlocks are portable locks with a shackle that may be passed through an opening (such as a chain link, or hasp staple) to prevent use, theft, vandalism or harm.

The term padlock is from the late fifteenth century. The prefix pad- is thought to be related to the Latin ped which may refer to the portability of a padlock; it is combined with the noun lock, from Old English loc, related to German loch, "hole".

There are padlocks dating to the Roman Era, 500 BC – 300 AD. They were known in early times by merchants traveling the ancient trade routes to Asia, including China.

Padlocks have been used in Europe since the middle La Tène period, subsequently spreading to the Roman world and the Przeworsk and Chernyakhov cultures. Roman padlocks had a long bent rod attached to the case, along with a shorter piece that could be inserted into the case. Przeworsk and Chernyakhov padlocks had a sleeve attached to the case, and a long bent rod which could be inserted into the case and the sleeve.

Padlocks have been used in China since the late Eastern Han dynasty (25–220 AD). According to Hong-Sen Yan, director of the National Science and Technology Museum, early Chinese padlocks were mainly "key-operated locks with splitting springs, and partially keyless letter combination locks". Padlocks were made from bronze, brass, silver, and other materials. The use of bronze was more prevalent for the early Chinese padlocks.

Padlocks with spring tine mechanisms have been found in York, England, at the Jorvik Viking settlement, dated 850 AD.

Smokehouse locks, designed in England, were formed from wrought iron sheet and employed simple lever and ward mechanisms. These locks afforded little protection against forced and surreptitious entry. Contemporary with the smokehouse padlocks and originating in the Slavic areas of Europe, "screw key" padlocks opened with a helical key that was threaded into the keyhole. The key pulled the locking bolt open against a strong spring. Padlocks that offered more key variance were the demise of the screw lock. Improved manufacturing methods allowed the manufacture of better padlocks that put an end to the Smokehouse around 1910.

Around the middle of the 19th century, "Scandinavian" style locks, or "Polhem locks", invented by the eponymous Swedish inventor Christopher Polhem, became a more secure alternative to the prevailing smokehouse and screw locks. These locks had a cast iron body that was loaded with a stack of rotating disks. Each disk had a central cutout to allow the key to pass through them and two notches cut out on the edge of the disc. When locked, the discs passed through cut-outs on the shackle. The key rotated each disk until the notches, placed along the edge of each tumbler in different places, lined up with the shackle, allowing the shackle to slide out of the body. The McWilliams company received a patent for these locks in 1871. The "Scandinavian" design was so successful that JHW Climax & Co. of Newark, New Jersey continued to make these padlocks until the 1950s. Today, other countries are still manufacturing this style of padlock.