Lydian is an extinct Indo-European Anatolian language spoken in the region of Lydia, in western Anatolia (now in Turkey). The language is attested in graffiti and in coin legends from the late 8th century or the early 7th century to the 3rd century BCE, but well-preserved inscriptions of significant length are so far limited to the 5th century and the 4th century BCE, during the period of Persian domination. Thus, Lydian texts are effectively contemporaneous with those in Lycian.

Strabo mentions that around his time (1st century BCE), the Lydian language was no longer spoken in Lydia proper but was still being spoken among the multicultural population of Kibyra (now Gölhisar) in southwestern Anatolia, by the descendants of the Lydian colonists, who had founded the city.

In 1916 the Sardis bilingual inscription, a bilingual inscription in Aramaic and Lydian allowed Enno Littmann to decipher the Lydian language. From an analysis of the two parallel texts, he identified the alphabetic signs, most of them correctly, established a basic vocabulary, attempted translation of a dozen unilingual texts, gave an outline of Lydian grammar, and even recognized peculiar poetical characteristics in several texts. Eight years later William Hepburn Buckler presented a collection of 51 inscriptions then known. The 109 inscriptions known by 1986 have been treated comprehensively by Roberto Gusmani; new texts keep being found from time to time.

All but a few of the extant Lydian texts have been found in or near Sardis, the Lydian capital, but fewer than 30 of the inscriptions consist of more than a few words or are reasonably complete. Most of the inscriptions are on marble or stone and are sepulchral in content, but several are decrees of one sort or another, and some half-dozen texts seem to be in verse, with a stress-based meter and vowel assonance at the end of the line. Tomb inscriptions include many epitaphs, which typically begin with the words 𐤤𐤮 𐤥𐤵𐤫𐤠𐤮 es wãnas ("this grave"). The short texts are mostly graffiti, coin legends, seals, potter's marks, and the like. The language of the Ionian Greek poet Hipponax (sixth century BCE, born at Ephesus) is interspersed with Lydian words, many of them from popular slang.

Lydian can be officially studied at Marburg University, Germany, within the Hittitology minor program.

Within the Anatolian group, Lydian occupies a unique and problematic position. One reason is the still very limited evidence and understanding of the language. Another reason is a number of features that are not shared with any other Anatolian language. It is still not known whether those differences represent developments peculiar to pre-Lydian or the retention in Lydian of archaic features that were lost in the other Anatolian languages. Until more satisfactory knowledge becomes available, the status of Lydian within Anatolian remains a "special" one.

The Lydian script, which is strictly alphabetic, consists of 26 signs:

The script is related to or derived from that of Greek as well as its western Anatolian neighbours, the exact relationship still remaining unclear. The direction of writing in the older texts is either from left to right or right to left. Later texts show exclusively the latter. Use of word-dividers is variable. The texts were found chiefly at the ancient capital of Sardis and include decrees and epitaphs, some of which were composed in verse; most were written during the 5th century and the 4th century BCE, but a few may have been created as early as the 7th century.