We demonstrate operation of a Ramey-mode Rb vapor-cell atomic clock based on a microwave cavity realized by additive manufacturing (3D-printing). The cavity design is based on a loop-gap approach and its critical electrode structure is realized in one monolithic piece by stereolithography of a polymer which simplifies the assembly of the cavity. The microwave magnetic field in the cavity shows excellent uniformity and high homogeneity across the Rb cell, which results in high-contrast Ramsey fringes observed on the clock transition. A measured clock stability of 2.2×10 -13 τ -1/2 demonstrates the feasibility of the approach. We discuss aging studies performed on 3D-printed test samples that are of relevance for long-term operation of the clock.