Count Adolf I of Nassau-Siegen (1362 – 12 June 1420), , was since 1384 Count of Diez, through his first marriage. With his brothers, he succeeded his father in 1416 as Count of Nassau-Siegen (a part of the County of Nassau), and also inherited the County of Vianden in 1417. He descended from the Ottonian Line of the House of Nassau.
Biography
Adolf was born in 1362 as the eldest son of Count John I of Nassau-Siegen and Countess .
On 30 November 1376, a marriage contract was concluded between Adolf's father and Count Gerhard VII of Diez, for a marriage of the latter's only daughter Jutta to Adolf. The family of the counts of Diez was on the verge of becoming extinct in the male line; the County of Diez would then pass to Jutta, who was at most eight years old. According to this contract, Adolf would immediately receive everything that was or would become due to his father from the inheritance of Nassau-Hadamar. In addition, Adolf was to receive the dowers belonging to his grandmother Adelaide of Vianden (the Herbermark and the parish of Haiger), as well as half of castle, city and district of Löhnberg, but only until he came into full possession of the Nassau-Hadamar inheritance. The marriage contract further stipulated that the of Nentershausen, Meudt, Salz, Hundsangen, and two others, transferred by Gerhard, and the district of were to be redeemed by John and given to his daughter-in-law as a dower. The dowry was set at 7,000 guilders and assigned to , the six Zents, Laurenburg, the Esterau and the Zents of Diez in the parish of Driedorf. If Gerhard were to die without leaving any sons, his daughter's marriage to Adolf would be consummated immediately and the entire County of Diez would pass to him.
Adolf came into possession of the County of Diez through his marriage to Jutta before 1384. Roman King Wenceslaus invested him with the county in 1384. The beginning of his reign was very unsettled: it fell at the time of a great devastating feud, which was led by the cities of the Rhenish Confederation and their allies, especially Count Rupert 'the Bellicose' of Nassau-Sonnenberg, against the nobility on the Rhine and in the Wetterau and their allies, to whom Adolf's father-in-law Gerhard belonged, under the name of the Hattsteinischen Krieg (Hattstein War). A claim by Adolf to the city of Frankfurt around 1393 for compensation for damages shows that the Frankfurters and Rupert in particular had caused great devastation in the and had burned several villages in the County of Diez itself. This was probably the reason why Adolf lived in constant hostilities with Frankfurt until 1412, as proven by several letters of refusal still in existence. That this very feud was the cause of the alliance which Adolf established with Archbishop Werner of Trier on 8 November 1388 and for which he had the latter pay him 2,000 guilders, is probable, because Adolf, from the relatives of the House of Nassau, against whom this alliance was not supposed to be directed, explicitly excluded Rupert.
As Count of Diez, Adolf build in 1394. The castle primarily served to secure Nassau-Diez territory against the counts of Katzenelnbogen (in particular Burgschwalbach).
With his brothers John II 'with the Helmet', Engelbert I and John III 'the Younger', Adolf already agreed on a joint continuation of the government of the County of Nassau-Siegen, when their father would die, on 21 December 1409. Whichever of the brothers would be native or closest to his lands on the father's death should take possession of them in all brothers' name until a division would have taken place. Likewise, they promised each other, out of conviction of mutual advantages, not to inflict any violent advances on each other, and to allow the preferences of one and the other to apply, which had been established by older decrees. Whoever would take something for himself alone would be disinherited. All parental decrees favouring one brother over the other were declared null and void in advance. Adolf hereby tacitly renounced his right to the part of Nassau-Hadamar and the districts of Herborn, Haiger and Löhnberg, which he could have claimed in advance from the marriage contract with the heiress of the County of Diez. In accordance with this agreement, the brothers took over the government jointly after their father's death in 1416. However, the intended division did not take place: Adolf had no male offspring, the elder John was not married, the younger of the same name was a clergyman; it was to be expected that a division would not last long. Together, the brothers bought back the other half of the city of Siegen from the Electorate of Cologne.