Podlachia, also known by its Polish name Podlasie (Polish: [pɔˈdlaɕɛ] ^ⓘ; Lithuanian: Palenkė; Belarusian: Падляшша, romanized: Padliashsha), is a historical region in north-eastern Poland. Its largest city is Białystok, whereas the historical capital is Drohiczyn.

Similarly to several other historical regions of Poland, e.g. Greater Poland, Lesser Poland, Mazovia, Pomerania, Silesia, Warmia, Podlachia possesses its own folk costumes, unique traditional architecture and cuisine. Between 1513 and 1795 it was a voivodeship with the capital in Drohiczyn. Now the part north of the Bug River is included in the modern Podlaskie Voivodeship with the capital in Białystok, whereas southern parts are located in the Masovian and Lublin Voivodeships.

The region is called Podlasie, Podlasko or Podlasze in Polish, Palenkė in Lithuanian, Padliašša (Падляшша) in Belarusian, Podljas’e (Подлясье) in Russian, פּאָדליאַשע Podlyashe in Yiddish, and Podlachia in Latin.

There are two hypotheses regarding the origin of the name of the region. According to the first one, the name is derived from the Polish word las 'forest', and means 'near the forest'. However, this hypothesis conflicts with historical phonology; it fails to explain the vocalism and especially the -ch-/-š- in Slavic languages and the -nk- in Lithuanian.

According to the second hypothesis, the name is derived from the word liakh (or lach, Ukrainian: лях, 'Pole'), and means 'near Poland'. The second hypothesis holds that the term comes from the expression pod Lachem, which may be translated literally as 'under the Poles' (see: Lechia). Some claim it to mean 'under Polish rule', though in the Middle Ages Podlachia was only partially under Polish rule, and since 1446 until 1569 the area belonged to the Grand Duchy of Lithuania. A better variant of this theory holds that the name originates from the period when the territory was within the Trakai Voivodeship of the Grand Duchy of Lithuania, along the borderline with the Mazovia, primarily a fief of the Poland of the Piasts and later on part of the Kingdom of Poland of the Jagiellons. The origin of this name is apparently in East Slavic, probably Old Ruthenian ljax, as the descendants of the Proto-Slavic word *lęxъ 'Pole' are most widespread there; there is no trace of nasalisation as would be expected in a native Polish word, but instead the typically East Slavic reflex -ja-, betraying the non-Lechitic origin.

Podlachia is located along the middle stretch of the Bug River between Mazovia in the west, Polesia and Volhynia in the east, the Narew River in the north and the Chełm Land in the south. The borders of Podlachia changed with time and was not the same as historical Podlaskie Voivodeship. Podlachia is sometimes divided into two parts (southern and northern), which had different administrative subordination.

Traditional capital of Podlachia is Drohiczyn that lies into northern and southern parts. The former is included in the modern-day Podlaskie Voivodeship with its capital at Białystok (the historical boundary goes exactly through the city). Sometimes, Siedlce has been considered the capital of the region.

The coat of arms of Podlachia was introduced in 1569. It was created by combining the coats of arms of Poland (white eagle albeit without a crown) and Lithuania (mounted armoured knight holding a sword and shield with the Jagiellonian Double Cross).