Unterbäch is a municipality in the district of Raron in the canton of Valais in Switzerland.
Unterbäch is first mentioned about 1280 as Underbechque. On 18 August 1957, Unterbäch was the first community in Switzerland to establish the communal voting and election rights for women.
Unterbäch has an area, as of 2011, of 22 square kilometers (8.5 sq mi). Of this area, 27.7% is used for agricultural purposes, while 27.7% is forested. Of the rest of the land, 1.6% is settled (buildings or roads) and 43.0% is unproductive land.
The municipality is located in the Westlich Raron district, on a high terrace above the left bank of the Rhone river.
The blazon of the municipal coat of arms is Gules, a Patriarchal Cross Or issuant from Coupeaux Vert.
Unterbäch has a population (as of December 2014) of 404.As of 2008, 3.1% of the population are resident foreign nationals. Over the last 10 years (2000–2010 ) the population has changed at a rate of -9.7%. It has changed at a rate of -6.2% due to migration and at a rate of -3.8% due to births and deaths.