Registered mail is a postal service in many countries which allows the sender proof of mailing via a receipt and, upon request, electronic verification that an article was delivered or that a delivery attempt was made. Depending on the country, additional services may also be available, such as:

Traditionally, registered mail was a manual process which gave rise to a great variety of distinctive postal markings, like handstamps, and usage of registration labels. Many countries have issued special postal stationery and postage stamps for registered mail. Earlier similar services were known as Money Letters. Today, the registration process is largely computerized with barcode registration labels replacing the traditional analog labels having only a printed serial number.

Generally, the item is pre-paid with the normal postage rate and an additional charge known as a registration fee. Upon payment of this fee the sender is given a receipt, and (usually) a unique numbered registration label is affixed to the letter. As the letter travels from post office to post office and through any sorting office, it has to be signed for on a ledger. This process is completed when the letter is delivered and the receiver signs for the item. With computerization and barcode technology, much of the logging once done manually has become simpler and leads to greater options for the sender and receiver alike to access the status of their shipment via the internet. Many postal authorities provide tracing information for registered items on their website.

Internationally, the use of registered mail requires labels with a 13-digit reference number and corresponding barcode (UPU S10). The first two letters indicate registration (usually "RR") while the last two letters usually represent the country where the registered item was posted. E.g., RR913282511SG indicating Singapore, RB5584847749CN indicating China or RR123456785KR indicating South Korea.

The earliest reference to a mail registration system dates to July 1556, during the reign of Mary Tudor, of England. In that example, the poste between this and the Northe should eche of them keepe a booke and entrye of every letter that he shall receive, the tyme of the deliverie thereof unto his hands with the parties names that shall bring it unto him, whose handes he shall also take to his booke, witnessing the same note to be trewe. This was likely for state security rather than mail security. In 1603, another Order of Council was made whereby all letters had to be recorded. This system was, in effect, a registration system, although it applied to all items sent via the post.

William Dockwra's 1680s London Penny Post also recorded all details on letters accepted for onward transmission, but unlike the General Post Office, gave compensation for losses.

The registration of letters as known today was introduced in 1841 in Great Britain. The letter had to be enclosed within a large sheet of green paper. The green sheet was addressed to the Post Office where the recipient lived. The green sheet was then used as a receipt and was returned to the office of origin after delivery. On 1 July 1858 the green sheet was replaced by a green silk ribbon and shortly afterward by a green linen tape. In 1870 the tape was replaced by green string. On the introduction of postal stationery registration envelopes in 1878 the string was replaced by printed blue crossed lines. The blue crossed lines have survived on registered letters to the present day. ^{[disputed (for: outdated source)  – discuss]}

Canada Post's Registered Mail service provides the sender with a mailing receipt, and upon delivery of the item, with the delivery date and a copy of the signature of the addressee or the addressee's representative. Registered Mail may include lettermail, documents, valuables, and literature for the blind, but does not include parcels.