Montgomeryshire (Welsh: Sir Drefaldwyn 'the shire of Baldwin's town', or simply Maldwyn) was one of the thirteen counties of Wales that existed from 1536 until their abolition in 1974, later becoming a district of Powys from 1974 to 1996. It was named after its county town, Montgomery, which in turn was named after one of William the Conqueror's main counsellors, Roger de Montgomerie, who was the 1st Earl of Shrewsbury.

The area of what was Montgomeryshire, now constitutes the northern part of the county of Powys. The current area was 2,174 square km (839 square miles).

The largest town was Newtown, followed by Welshpool and Llanidloes.

The Treaty of Montgomery was signed on 29 September 1267, in the town of Montgomery, which had recently been established as an English incursion on the Welsh side of the border, to control a strategic border crossing. The surrounding region (on the Welsh side of the border) otherwise comprised the mediaeval principality of Powys Wenwynwyn, the southern of the two states into which the Kingdom of Powys had been divided a century before.

Attacks by Gwynedd on Powys Wenwynwyn led the latter to seek the assistance of the English. Ultimately this led them to convert their territory into a marcher lordship, via surrender and regrant, as a way to strengthen their position; the ruling princes of Powys Wenwynwyn became the Lords of Powys, feudally bound to the English king, and able to fully rely on English backing, but otherwise independent. The prince took an English-style surname - Owen de-la Pole - after his capital city, Pool (now Welshpool).

With the introduction of the Laws in Wales Acts 1535–1542 the marcher lordships were converted into English counties. The Lordship of Powys – the former Powys Wenwynwyn – became Montgomeryshire; the county town being Montgomery, the strongest centre of English authority in the region, rather than centre of Welsh authority, Welshpool. Montgomeryshire was thus ultimately formed from the cantrefi of Powys Wenwynwyn:

In addition, for practical reasons, Montgomeryshire gained the commote of Ceri, which had formed a northwards spur of the less organised region Between Wye and Severn; most of the rest of the latter region became Radnorshire.

Montgomeryshire was bordered, to the north, by Denbighshire, to the east and south east by Shropshire, to the south by Radnorshire, to the south west by Cardiganshire, and to the west and north west by Merionethshire. When, in subsequent centuries, the concept of Wales was once again officially distinguished from England, all of these counties were deemed Welsh, except for Shropshire. Montgomeryshire was the birthplace of Welsh Catholic martyr Saint Richard Gwyn (in 1537).