Montgomeryshire ( Welsh: Sir Drefaldwyn ) is one of the thirteen traditional counties and a former administrative county of Wales. Historical capital is the city of Montgomery. Since 1974, Montgomeryshire is a part of Powys.
The landscape is predominantly mountainous. The highest elevation is the Cadair Berwyn (830 m). The Severn and the Dyfi are the main rivers. Significant places besides Montgomery are Llanfyllin, Machynlleth, Llanidloes, Newtown and Welshpool.
In 1961 the county had 44 228 inhabitants in 2064 km ².
The county is named after Roger de Montgomerie, a follower of William the Conqueror.
Until 1974 Montgomeryshire was a separate administrative county and then next Brecknockshire and Radnorshire one of the three districts of the new county of Powys. 1996, all districts were dissolved and Powys has been a unitary authority with the status of a County. At the 2001 census, the area had the former Montgomeryshire District 59 474 inhabitants.
The Montgomeryshire constituency of the British House has continuously gained up to one term over 100 years by the Liberals.