We have previously developed side-viewing endoscopic OCT systems to detect colorectal cancer in the murine model, which longitudinally scans the mouse colon at 8-16 discrete angular positions. This small number of angles is chosen to keep imaging time and the amount of data to analyze reasonable, but this azimuthal undersampling of the tissue may result in missed or incorrectly characterized adenomas. A need exists for a spiral-scanning OCT endoscope capable of generating 3D, in vivo OCT data sets that satisfy the Nyquist criterion for adequate sampling of the tissue. Our new endoscopic system replaces the sample arm optics of a commercial OCT system with a spiral-scanning, gradient-index lens-based endoscope. The endoscope provides unit magnification at a working distance capable of producing a focal depth of 280 μm in tissue. The working distance accounts for a 41° rod prism that reflects the beam through the endoscopic window into the tissue while minimizing back reflection. A swept-source laser with a central wavelength of 1040 nm and spectral bandwidth of 80 nm provides an axial resolution of 12 μm in air and 9 μm in water. The endoscope has a theoretical diffraction-limited lateral resolution of 5.85 μm. We present fully sampled, 3D, in vivo images of the mouse colon.