ISO/IEC 8859-1:1998, Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1987. ISO/IEC 8859-1 encodes what it refers to as "Latin alphabet no. 1", consisting of 191 characters from the Latin script. This character-encoding scheme is used throughout the Americas, Western Europe, Oceania, and much of Africa. It is the basis for some popular 8-bit character sets and the first two blocks of characters in Unicode.

ISO-8859-1 was (according to the standard, at least) the default encoding of documents delivered via HTTP with a MIME type beginning with "text/" (HTML5 changed this to Windows-1252). As of January 2023, 1.4% of all (and only 16 of the top 1000) web sites use ISO/IEC 8859-1. It is the most declared single-byte character encoding in the world on the Web, but as Web browsers interpret it as the superset Windows-1252, the documents may include characters from that set.

Depending on the country or language, use (on websites at least) can be much higher than the global average, e.g. (including Windows-1252), for Brazil according to website use, use is at 9.2%, and in Germany at 3.9%.

ISO-8859-1 was the default encoding of the values of certain descriptive HTTP headers, and defined the repertoire of characters allowed in HTML 3.2 documents, and is specified by many other standards. It's rarely assumed to be the encoding of text in operating systems (while it was very common in the past), though if an 8-bit encoding is used then its superset encoding Windows-1252 is most likely to be used, on Microsoft Windows if there is no byte order mark (BOM); this is only gradually being changed to UTF-8.

ISO-8859-1 is the IANA preferred name for this standard when supplemented with the C0 and C1 control codes from ISO/IEC 6429. The following other aliases are registered: iso-ir-100, csISOLatin1, latin1, l1, IBM819. Code page 28591 a.k.a. Windows-28591 is used for it in Windows. IBM calls it code page 819 or CP819 (CCSID 819). Oracle calls it WE8ISO8859P1.