Lordship of Grenzau
Herrschaft Grenzau
Arms-Isenburg-Loweldsal Arms-Isenburg-Lowyousal
1158 - 1866

Grenzau in muted orange
Established 1158
Mediate to Trier 1361
To Nassau-Beilstein 1439
To Trier 1446
To Lower Isenburg 1460
Extinct; to Trier 1664
To Nassau 1803
To Prussia; Abolished 1866

The Lordship of Grenzau was an estate of the Holy Roman Empire based around Burg Grenzau in Rhineland-Palatinate, Germany. The castle of Grenzau was constructed by Count Henry I of Isenburg-Grenzau from c. 1208 until 1215. In 1328 the Lordship was part a territory of the Archbishops of Trier.

The castle was first mentioned in 1213. Henry gave the castle the name "Gransiole" (meaning "Great Joys" in Old French), which slowly developed into Grensove (Grensauwe), Grentzawe, Grenz, and finally Grenzau. In 1263 Henry's son and heir Henry II of Isenburg-Grenzau was mentioned as the "Lord of Grenzau". In the following decades Grenzau became a centre of the Lower Westerwald.

In 1286, one year before his death, Henry partitioned his realm between his three sons. The eldest Eberhard received Grenzau, while his brothers Louis and Gerlach I received the territories around Büdingen and Hönningen respectively. Eberhard died in 1290 and Grenzau was inherited by Louis. Louis' heir Lothar's realm was divided by his sons after his death, with Grenzau passing to his son Philip I who founded the second line of Isenburg-Grenzau.

The expansionistic policy of Baldwin of Luxembourg, the Archbishop of Trier, expanded the secular borders of the prince-archbishopric deep into the Westerwald. After relatively minor feuds, he organised Grenzau as an administrative centre of the archdiocese, and acquired half the rulership of the lordship in 1346. The following year, Count Philip expelled the new burgraves from Grenzau, leading to the Grenzovian Feud. Out of 800 men from Coblenz and an army of the archbishop's, only 172 survived the retreat back from battle against Philip and his ally, Count Reinhard I of Westerburg. It was only King Louis IV of Upper Bavaria who intervened and ceased hostilities in 1350.

In 1361 the coadjutor of Trier, Kuno of Falkenstein, forced Philip to accept mediate lordship under the archbishopric. After his death later that year, his successors Eberhard (1361 - 1399) and Philip II (1399 - 1439) had largely uneventful reigns in which the castle of Grenzau was rebuilt. In 1439 after Philip's death, Grenzau was inherited by Count Philip of Nassau-Beilstein. After Philip's death in 1446, the archbishops took the entire lordship for themselves.

In 1460 Count Gerlach II of Lower Isenburg purchased half the lordship of Grenzau. From 1495 the House of Lower Isenburg became one of the most important and respected families of the day after Gerlach III was selected to hold the Emperor's banner at the Reichstag of Worms in 1495. In 1502 after the death of Count Gerlach II, Lower Isenburg was partitioned into Isenburg-Grenzau and Isenburg-Neumagen, with Grenzau passing to Gerlach III]]. Gerlach was succeeded by his son Henry the Elder in 1530. Henry married the heiress Margaret of Wertheim, who brought to Isenburg-Grenzau many territories in the Odenwald, the Lower Rhine, and Lorraine.

Henry's son John, who succeeded him in 1554, was elected the Archbishop of Trier in 1547. In 1550 the castle of Grenzau was rebuilt and modernised with 15 cannons, and was (and is) the only castle of its kind in the Rhineland. In 1567 Salentin VII was elected the Archbishop of Cologne. He was an important supporter of the Counter-Reformation and used his position there to improve conditions in Grenzau, making it an immediate estate in the Imperial Diet. Salentin retired from the church in 1577 to take up his duties as Count of Isenburg-Grenzau.

The last Count of Isenburg-Grenzau was Ernest (1619 - 1664), who was a field captain of the Imperial army during the Thirty Years' War. The Swedes plundered Grenzau in 1637. After his death in 1664 at age 80, the Lower Isenburg line was extinct. Although the Upper Isenburgish line was extant, Grenzau passed to the Archbishopric of Trier. Thereafter, the lordship of Grenzau declined in importance and was largely forgotten. With the secularisation of Trier in 1803, the lordship passed to the Nassau-Weilburg. In 1806, it was organised as part of the Duchy of Nassau. After the Austro-Prussian War in 1866, Nassau was conquered by Prussia who abolished the old territorial divisions.