Martinstein is a municipality located in the Bad Kreuznach district of Rhineland-Palatinate, Germany. It functions as an Ortsgemeinde within the Verbandsgemeinde of Bad Sobernheim, which has its seat in the town of Bad Sobernheim.
The village is classified as a state-recognized tourism resort. Despite its status, it covers an area of only 39 hectares, making it the smallest municipality by land area in Germany.