Collagenous tissues (e.g. bone and tendon) have well organized hierarchical structures. To improve understanding of the mechanical behavior of collagenous tissues and to guide the development of multiscale models, mechanical testing at different length scales is required. Whole tissues, fascicles, and fibril bundles have been studied extensively, but little is known at the fibrillar level. Using microelectromechanical systems (MEMS) technology, tensile mechanical testing was performed on type I collagen fibril specimens isolated from the dermis of sea cucumbers. In air uniaxial tensile tests showed that the fibrils had a small strain elastic modulus of 860 ± 450 MPa, a yield stress of 220 ± 140 MPa, and a yield strain of 21% ± 13%. In vitro fracture tests showed that the fibrils had an elastic modulus of 470 ± 410 MPa, a fracture strength of 230 ± 160 MPa, and a fracture strain of 80% ± 44%. The fibrils displayed significantly lower elastic modulus in vitro than in air. Both the fracture strength/strain obtained in vitro and in air were significantly larger than those obtained in vacuo, indicating that the difference arises from the lack of intrafibrillar water molecules produced by vacuum drying. Fracture strength/strain of fibril specimens were different from those reported for collagenous structures of higher hierarchical levels, indicating the importance of obtaining these properties at the fibrillar level for multiscale modeling. In vitro coupled creep and stress relaxation tests demonstrated the intrinsic viscoelastic behavior of collagen fibrils. The stress-strain-time data were fitted using a Kelvin model consisting of a spring and a dashpot in parallel. The fibrils showed an elastic modulus of 180 ± 100 MPa, a viscosity of 4.7 ± 3.2 GPa*sec, and a relaxation time of 29 ± 16 sec. The fibrillar relaxation time was smaller than the tissue-level relaxation time, suggesting tissue relaxation is dominated by non-collagenous components (e.g. proteoglycans). To our knowledge, in vitro fracture and viscoelastic properties of isolated collagen fibrils were measured for the first time. The mechanical properties obtained in this work can be used as input parameters for multiscale modeling and help guide the development of synthetic biomaterials.