Micropiles are being increasingly used in existing structures for foundation rehabilitation and in new structures to enhance their seismic safety. For the seismic design of piles in sands, numerical models of piles under lateral loading using the p-y relationships proposed by the American Petroleum Institute (API) are widely used. However, the validity of this approach has been questioned by many researchers. In this regard, this study presents the development of experimental and numerical p-y relationships for shallow layers (depths less than 1.0 m) of micropiles in fully saturated medium-dense sands using a quasi-static test. A steel pipe was embedded into the sands and fixed at the bottom of the soil container. Lateral loads were applied on the pile head using the displacement-control technique. A piecewise-function–based p-y derivation method was adopted to develop the experimental p-y curves. Distributions of bending moment, shear force, soil reaction, rotation, and deflection along the pile are presented. Subsequently, bilinear shallow-layer p-y relationships are proposed. Test results show that experimental p-y curves for micropiles at the shallow depths are more flexible (lower initial stiffness) and stronger (greater strength) than those recommended by the API. The quasi-static test is then simulated using either the proposed bilinear p-y relationships or those recommended by the API. Results show that the global force-displacement relationship, bending moment distribution and lateral deflection of piles are accurately predicted by the proposed shallow-layer p-y relationships. By contrast, the conventional API p-y relationship reasonably predicts the lateral strength and maximum bending moment while overly predicting the depth-to-maximum bending moment of micropiles in saturated medium-dense sands.