The siege of Paris of 845 was the culmination of a Viking invasion of West Francia. The Viking forces were led by a Norse chieftain named "Reginherus", or Ragnar, who tentatively has been identified with the legendary saga character Ragnar Lodbrok (Old Norse: "Ragnarr Loþbrók", contemporary Icelandic: "Ragnar Loðbrók". The link to Ragnar Lothbrok seems tenuous at best). Reginherus's fleet of 120 Viking ships, carrying thousands of men, entered the Seine in March and sailed up the river.

The Frankish king Charles the Bald assembled a smaller army in response but after the Vikings defeated one division, comprising half of the army, the remaining forces retreated. The Vikings reached Paris at the end of the month, during Easter. They plundered and occupied the city, withdrawing after Charles the Bald paid a ransom of 7,000 French livres [2,570 kg (83,000 ozt)] in gold and silver.