Sigebert IV (c. 671 - c. 679) was the son of Dagobert II and a Saxon duchess called Mathildis (also called Mechthilde), and the grandson of Sigebert III of the Merovingian dynasty. He is believed to have died either just before or after his father and other members of the royal family, who were killed on the orders of Ebroin, the Frankish Mayor of the Palace of Neustria.
In the 10th-century biography of Saint Arbogast, Vita Sancti Argobasti, Sigebert is mentioned but not named as having died during a hunting accident and later miraculously raised from the dead by Arbogast, Bishop of Strasbourg and close friend of Dagobert II.
Priory of Sion hoax
According to the pseudohistorical Dossiers Secrets d'Henri Lobineau and related documents, Sigebert IV, on the assassination of his father Dagobert II, was rescued by his sister and smuggled to the domain of his mother the (otherwise unknown) Visigoth princess, Giselle de Razès in Rennes-le-Château. He is said to have arrived in the Languedoc in 681 and, at some point, adopted or inherited his uncle's titles, duke of Razès and count of Rhedae. He is also said to have adopted the surname, or nickname, of “Plant-Ard” (subsequently Plantard) from the French appellation ‘rejeton ardent’ ‘ardently flowering shoot’ of the Merovingian vine. Under this name, and under the titles acquired from his uncle, he is said to have perpetuated his lineage.
Journalists and scholars who have debunked the Priory of Sion hoax argue that Pierre Plantard created the Dossiers Secrets d'Henri Lobineau and the fictitious biography of Sigebert IV to support his false claim that he was a descendant of the extinct Merovingian dynasty through this figure. In 1990, Plantard revised himself by claiming he was only descended from a cadet branch of the line of Dagobert II, while arguing that the direct descendant was really Otto von Habsburg, who, according to Plantard, was descended from Sigebert I (the son of Bera II and the grandson of Wamba), who married Magdala, the granddaughter of Dagobert II.
- According to the 8th century Vita Sancti Wilfrithi ( Life of St Wilfrid), and also mentioned in a version of the Vita Sancti Arbogasti.
- The early 8th century Vita Amandi Episcopi referred to "Sygiberto"  and to "Sigebertum, Dagoberti Filium" 
- Historians tend to ignore the importance of these references, François Eudes de Mézeray, in Histoire de France (Paris, 1685) referred to an anonymous son, and doubted his existence.
- René Descadeillas, Mythologie du Trésor de Rennes: Histoire Veritable de L'Abbé Saunière, Curé de Rennes-le-Château, Annexe VI (Mémoires de la Société des Arts et des Sciences de Carcassonne, Annees 1971-1972, 4me série, Tome VII, 2me partie; 1974).
- Utho Argentinensis, Vita Sancti Arbogasti episcopi argentinensis in Germania prima (Apud Bolland., Junii tom. V, p. 170).  This version of the text does not refer to Mathildis.
- Christian Settipani, in La Prehistoire des Capetiens (1993) argued that the Vita Sancti Arbogasti is too late a source to be accepted without care, and points to evidence that it places Arbogast and his successor Florentius up to a century too late (Arbogast died in around 600). He argues that the trio of Dagobert II, and his alleged wife Mathildis and son Sigebert, may actually have arisen from confusion with Dagobert I, his wife Nanthilde and son Sigebert (his mother was actually Ragnetrude).
- Baigent, Michael; Leigh, Richard; Lincoln, Henry (1982). The Holy Blood and the Holy Grail. Corgi. ISBN 0-552-12138-X.
- Jean-Luc Chaumeil, The Priory of Sion - Shedding Light on the Treasure and Legacy of Rennes-le-Château and the Priory of Sion (Avalonia, 2010).
- Quoting Pierre Plantard: "If anyone can claim to be a descendant of Sigisbert IV in the direct line it can only be Otto von Habsburg, and he alone. To all those people who write to me I have given this same reply." From Vaincre – Reprend le titre d'un périodique paru en 1942-1943, Number 1, April 1990 
- Quoting Plantard: "We would like to repeat that in no case have we found any trace of the son of Dagobert II in the list of the Visigothic Razes. This Sigibert IV found refuge with his abbess sister at Oeren and was the cousin of Sigebert de Rhedae, who was alive more or less around the same time. Historians conflate these two Sigiberts into one person. When did Sigebert IV die? We don't know. Some think that he was the founder of the Hapsburg family."
- Jean-Luc Chaumeil, Le trésor du Triangle d'Or, page 163 (Nice: Alain Lefeuvre, 1979).