Mailsort is a five-digit address-coding scheme used internally by the Royal Mail (the UK's postal service) for the automatic direction of mail. The first three digits, the Residue Selection Code, identify an area corresponding to one or more postcode districts. The last two digits, called the Direct Selection code, identify a postcode group within the district.

Mail users who can present mail sorted by Mailsort code (not necessarily down to the fifth digit) and in quantities of 4000 upwards receive a discounted postal rate.

Although the majority of people in the UK use the postcode, the mailsort code is now used for automated sorting. The postcode failed in its objective, partly because its inflexibility led to numerous initial two-letter codes being directed to a single main sorting office, and this led to some addresses having non-obvious characters. Furthermore, errors were introduced by the easy confusion by autotype reading of some characters of often hand-written addresses.