The Tati language (Tati: تاتی زبون, Tâti Zobun) is a Northwestern Iranian language spoken by the Tat people of Iran which is closely related to other languages such as Talysh, Zaza, Mazandarani and Gilaki.

Some sources use the term Old Azeri to refer to the Tati language as it was spoken in the region before the spread of Turkic languages, and is now only spoken by different rural communities in Iranian Azerbaijan (such as villages in Harzanabad area, villages around Khalkhal and Ardabil), and also in Zanjan and Qazvin provinces. Alongside with Tati dialects, Old Azeri is known to have strong affinities with Talysh and Zaza language. Tati, Zaza and Talysh are considered to be remnants of old Azeri. Harzandi dialect that thought to be descendant of the Old Azeri language was positioned between the Talysh and Zaza.

In any language, roots and verb affixes constitute the most basic and important components of a language. The root is an element included in all the words of a lexical family and carries the basic meaning of those lexical items. A verb affix is an element added to the root to form a new meaning. In many new Iranian languages, verb affixes have been left almost unnoticed, and it will be possible, by the act of deriving roots, to clear up most of their structural and semantic ambiguities. Unlike the root, verb affixes can be easily identified and described. In many languages, verb affixes act as the base of verb formation and are often derived from a limited number of roots. Tati, Talysh, Mazandarani and Gilaki languages belong to North-western Iranian languages currently spoken along the coast of Caspian Sea. These languages which enjoy many old linguistic elements have not been duly studied from a linguistic perspective.^{[original research?]}

In the field of phonetics Tati is similar to the rest of the north-western Iranian languages: it is distinguished by the persistence of Iranian *z, *s, *y-, * v- against the south-western d, h, j-, b-; development /ʒ/ < * j, */t͡ʃ/ against the south-west z, and the preservation of intervocalic and postvocalic *r and even, for a number of dialects, development rhotacism.

In the field of morphology, Tati is less analytical in structure than the south-western Iranian languages. Having lost the ancient foundations of classes and verb, tati preserved case (two case: direct, or subjective, and oblique). It has a grammatical gender feature in many dialects and exhibits two genders just like the Zaza.

Tati is, like Zaza of the same linguistic branch, an ergative language, i.e. "with transitive verbs the subject/agent of the verb is expressed by the direct case in the present tenses, but by the oblique in the past tenses, whereas the direct object/patient in the present tenses is expressed by the oblique, but by the direct in the past".
Khalkhali is one of the Tati dialects spoken in Shahrood and Xorsh-rostam districts of Khalkhal. Khalkhali Tati is distinguished from other dialects producing ergative structures, because of the adherence of verb to semantic object, in number, person and specially in gender. Meanwhile, according to some evidence in this dialect, apart from past transitive verbs, some intransitive verbs are influenced by the ergative structure.

The phonology is based on the Southern Tati dialects:

The following sounds /r, v, q/ may allophonically range to the sounds [ɾ, β, ʁ].