Hermann II (Slovene: Herman; early 1360s – 13 October 1435), Count of Celje, was a Styrian prince and magnate, most notable as the faithful supporter and father-in-law of the Hungarian king and Holy Roman Emperor Sigismund of Luxembourg.

Hermann's loyalty to the King ensured him generous grants of land and privileges that led him to become the greatest landowner in Slavonia.

He served as governor of Carniola, and twice as ban of the combined provinces of Slavonia, Croatia and Dalmatia, and was recognized by a treaty in 1427 as heir presumptive to the Kingdom of Bosnia.

The House of Celje's rise to power culminated in achieving the dignity of Prince of the Holy Roman Empire.

The death of his son-less cousin William on 19 September 1392 made him the sole possessor of the family titles and estates.

He quickly remarried to Veronika of Desenice, but Hermann refused to accept a minor petty noblewoman as his new daughter-in-law.

[3] In 1396, Hermann bravely fought at the side of King Sigismund of Hungary during the battle against the Ottoman Turks in the Bulgarian town of Nicopolis.

[1] In 1405, Sigismund married Hermann's youngest daughter Barbara and granted extensive lands in Slavonia to his father-in-law.

The marriage of his son Louis and Duke Ernest of Bavaria's daughter Beatrix provided Hermann with a powerful ally against his Habsburg overlords.

His goal was finally achieved in 1423 when Duke Ernest of Styria and Carinthia renounced his feudal supremacy over the counts of Celje.

[2] Now enjoying a direct legal relationship with the Crown, Hermann was free to concentrate on a new goal: becoming Prince of the Holy Roman Empire.

He was close to succeeding in this endeavour as well in 1430, but the draft charter granting him this honour never seems to have been published, possibly due to Habsburg objections.

The plan nevertheless went ahead; a treaty providing for Hermann's accession in the event of Tvrtko's death without male issue was signed on 2 September 1427.