This article describes the development of a nickel-catalyzed regio- and diastereoselective formal [3+2] cycloaddition between N-substituted indoles and donor–acceptor cyclopropanes to synthesize cyclopenta[b]indoles. Optimized reaction conditions provide the desired nitrogen-containing cycloadducts in up to 93% yield and dr 8.6:1 with complete regioselectivity. The substrate scope showed high tolerance to various substituted indoles and cyclopropanes, resulting in the synthesis of six new cyclopenta[b]indoles and the isolation of five derivatives previously reported in the literature. In addition, a mechanistic proposal for the reaction was studied through online reaction monitoring by ESI-MS, allowing for the identification of the reactive intermediates in the Ni(II) catalyzed process. X-ray crystallography confirmed the structure and relative endo stereochemistry of the products. This method enables the fast and efficient construction of fused indolines from readily accessible starting materials. © 2024 by the authors.Universidad Católica del Norte, UCN; Dirección General de Postgrado; Vicerrectoría de Investigación y Desarrollo Tecnológico; Agencia Nacional de Investigación y Desarrollo, ANID, (3190118, 3210175, 21221447); FONDECYT-ANID, (1220618, 1210576); VRIDT-UCN, (20230803020-VRIDT-UCN)