Siegfried I (c. 1010 – 7 February 1065) is considered the progenitor of the Carinthian ducal House of Sponheim (Spanheimer) and all of its lateral branches, including the Counts of Lebenau and the Counts of Ortenburg. He is documented as Count of Sponheim from 1044 and served as margrave of the Hungarian March in 1045/46 and as count in the Puster Valley and the Lavant Valley from 1048 until his death.

Siegfried was born at Sponheim Castle in Rhenish Franconia. Likewise Siegfried had a family relationship of unknown degree with Count Stephan I of Sponheim (d. ca. 1080), patriarch of the Rhenish branch of the Sponheim dynasty, which survives as the present-day Princes of Sayn-Wittgenstein.

In 1035 the Salian emperor Conrad II marched against the rebellious Duke Adalbero of Carinthia. In his attendance was Count Siegfried as his close companion, who thus arrived from Rhenish Franconia in the southeastern estates of the German kingdom. Adalbero was deposed and succeeded by the Salian duke Conrad the Younger in 1036.

Siegfried married Richgard, the heiress of Count Engelbert IV in the Puster Valley from the Carinthian family of the Sieghardinger and Aribonids. Through the marriage with Richgard, he obtained large possessions in Tyrol and also in Carinthia, for example the Lavant Valley (in modern Austria) and probably also Laško and some other parts in the March of Carniola (in modern Slovenia) like the territories around Ljubljana.

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