Despite the recent advances in automatic methods for computing skinning weights, manual intervention is still indispensable to produce high quality character deformation. However, current modeling software does not provide efficient tools for the manual definition of skinning weights. Widely used paint-based interfaces gives users high degrees of freedom, but at the expense of significant efforts and time. This paper presents a novel interface for editing skinning weights based on splines, which represent the isolines of skinning weights on a mesh. When a user drags a small number of spline anchor points, our method updates the shape of the isolines and smoothly interpolates or propagates the weights while respecting the given iso-value on the spline. We introduce several techniques to enable the interface to run in real-time, and propose a particular combination of functions that generates appropriate skinning weight over the surface. Users can create skinning weights from scratch by using our method. In addition, we present the spline and the gradient fitting methods that closely approximate initial given weights, so that a user can modify the weights with our spline interface. We show the effectiveness of our spline-based interface through a number of test cases.