Type III collagen is a major fibrillar collagen consisting of three identical α₁(III)-chains that is particularly present in tissues exhibiting elastic properties, such as the skin and the arterial wall. Heterozygous mutations in the COL3A1 gene result in vascular Ehlers-Danlos syndrome (vEDS), a severe, life-threatening disorder, characterized by thin, translucent skin and propensity to arterial, intestinal and uterine rupture. Most human vEDS cases result from a missense mutation substituting a crucial glycine residue in the triple helical domain of the α₁(III)-chains. The mechanisms by which these mutant type III collagen molecules cause dermal and vascular fragility are not well understood. We generated a transgenic mouse line expressing mutant type III collagen, containing a typical helical glycine substitution (p.(Gly182Ser)). This Col3a1^Tg-G182S mouse line displays a phenotype recapitulating characteristics of human vEDS patients with signs of dermal and vascular fragility. The Col3a1^Tg-G182S mice develop severe transdermal skin wounds, resulting in early demise at 13–14 weeks of age. We found that this phenotype was associated with a reduced total collagen content and an abnormal collagen III:I ratio, leading to the production of severely malformed collagen fibrils in the extracellular matrix of dermal and arterial tissues. These results indicate that expression of the glycine substitution in the α₁(III)-chain disturbs formation of heterotypic type III:I collagen fibrils, and thereby demonstrate a key role for type III collagen in collagen fibrillogenesis in dermal and arterial tissues.