We examine the BRS invariance of the open pure spinor superstring in the presence of background superfields on a Dp-brane. It is shown that the BRS invariance leads not only to boundary conditions on the spacetime spinors, but also to supersymmetric DBI equations of motion for the background superfields on the Dp-brane. These DBI equations are consistent with the supersymmetric DBI equations for a D9-brane. background fields should satisfy full non-linear equations of motion for a supersymmetric DBI action. Non-abelian extension of this formalism is discussed in [7] as the boundary fermion formalism where Chan-Patton factors describing coincident D-branes are replaced by boundary fermions * . Unlike the formulations mentioned above, the pure spinor formulation [23] enables us to quantize a superstring in a super-Poincaré covariant manner. In this formulation, the κ-symmetry in the GS formulation is replaced with the BRS symmetry. It is shown in [24], correspondingly to the κ-symmetry analysis [6], that the classical BRS invariance of an open pure spinor superstring leads to supersymmetric DBI equations of motion on a D9-brane, which have the non-linear N = 1 supersymmetry as well as the manifest N = 1 supersymmetry. These equations precisely coincide with those obtained in the superembedding formalism [25]. Furthermore, the non-abelian extension of supersymmetric DBI equations is proposed. In [26] (see also [27] and [28]), D-brane boundary states are constructed in the pure spinor formulation. Especially, calculating the disk scattering amplitude suggests that the coupling of the boundary state to the background fields will reproduce the DBI kinetic term and the Wess-Zumino term of the D9-brane effective action. These achievements might imply the fact that the low-energy effective theory on the D9-brane is determined uniquely by the ten-dimensional N = 2 supersymmetry.In this paper we will derive supersymmetric DBI equations of motion on a Dp-brane, as well as a D9-brane, from the BRS invariance of the open pure spinor superstring. Our approach is similar to that taken in [24] for a D9-brane. However the inclusion of Dirichlet components requires improvements which are not just a dimensional reduction of the case of a D9-brane. As in [24], we will provide two boundary terms, the counter term S b for the N = 1 supersymmetry transformation of the world-sheet action S 0 and the background superfield coupling V as a relevant extension of the pure spinor vertex operator. It is found that the contribution of Dirichlet components in them cannot be determined unless considering the BRS invariance. In [24], by using non-trivial boundary conditions given by the general variation of S 0 +S b +V , the BRS charge conservation leads to supersymmetric DBI equations for the D9-brane. On the other hand, we will show that supersymmetric DBI equations in diverse dimensions are extracted only from the BRS transformation of S 0 + S b + V under an identification on the D-brane position.This paper is organized as follows. In section 2,...