Werdenberg was a county of the Holy Roman Empire situated on either side of the Rhine, including parts of what is now St. Gallen (Switzerland), Liechtenstein, and Vorarlberg (Austria). It was partitioned from Montfort in 1230. In 1260, it was divided into Werdenberg and Sargans.
It is named for Werdenberg Castle, today located in the municipality of Grabs in the Swiss canton of St. Gallen, seat of the counts of Werdenberg (Werdenberger), The family was descended from count Hugo II of Tübingen (d. 1180), who married Elisabeth, daughter of the last count of Bregenz, thus inheriting substantial territory along the Alpine Rhine. His son was Hugo I of Montfort (d. 1228), whose son Rudolf I is considered the founder of the Werdenberg line. Rudolf's sons Hugo I of Werdenberg-Heiligenberg and Hartmann I of Werdenberg divided the southern territory of the Montfort inheritance, establishing the two lines of Werdenberg-Heiligenberg and Werdenberg-Sargans.
In 1308 Werdenberg was further divided into Werdenberg-Heiligenberg (Linzgau) and Werdenberg-Werdenberg. The Vaduz line of Counts of Werdenberg died out in 1406 and Vaduz passed to the Barons of Brandis.
The family fractured further into a number of cadet branches. The line of Werdenberg-Heiligenberg-Sigmaringen-Trochtelfingen remained influential in the early 16th century in the context of the Swabian League but was extinct in 1534.
The Werdenberg feud (Werdenbergfehde) was a major series of feuds between the Werdenberg and their neighbours in the late 15th century, most notably their conflict with the von Zimmern family of Swabia. The feud between the lords of Werdenberg and of Zimmern escalated in 1488, rising to an importance above merely regional concerns, influencing the imperial policy of Frederick III and Maximilian I regarding the formation of the Swabian League, the Imperial Reforms and the history of the Old Swiss Confederacy.