Wearable systems for gait analysis in daily living have been recently developed. Previous studies have demonstrated the significant potential of these systems; however, most of them focused on the level-walking condition, which is a limited portion of daily activities. To provide a new contribution to the gait analysis field, we have developed the first models for estimating three-dimensional (3D) ground reaction force (GRF) and center of pressure (CoP) during stair and slope ascent/descent with wearable sensors. Our system comprises light weight inertial measurement units (IMUs) and foot pressure sensors. We modeled the correlation between the measurements obtained with the wearable sensors and the ground truth of GRF/CoP from force plates, on the basis of linear regression models. Twenty healthy subjects completed a collection of ascent/descent tasks on stairs or slopes. We tested our models using cross-validation to evaluate the estimation accuracy in terms of the root mean square error (RMSE), the normalized RMSE (NRMSE), and the Pearson's correlation coefficient between the estimated GRF/CoP and those obtained from force plates. The experimental results showed practical estimation accuracy was obtained for GRF (RMSE ≤ 44.94 N) and CoP (RMSE ≤19.43 mm). Our system promises to contribute to clinical and sports medicine research by serving as a novel tool for assessing stair and slope ascent/descent outside laboratory environments.