The importance of the Ligament of Marshall (LOM) to rapid activations within the Left Superior Pulmonary Vein (LSPV) during AF remains poorly understood. We aimed to characterize the importance of electrical coupling between the LSPV with Left Atrium (LA) and LOM in the generation of high frequency activations within this PV. We performed high density mapping of the LSPV-LA-LOM junction in 8 dogs using 1344 electrodes with a 1-mm resolution before and after posterior ostial ablation to diminish PV-LA electrical connections. A LOM potential was recordable up to 6.5 ± 2.2 mm into the LSPV in all dogs during sinus rhythm (SR) and LA pacing. Functional LOM-LSPV electrical connections bypassing the PV-LA junction were present in 5/8 dogs. Direct LOM-LSPV connections contributed to 46.5 ± 16.0 % of LSPV activations during AF, resulting in a greater propensity to develop focal activations (P<0.05) and a higher activation rate during AF of LSPVs with direct LOM connections compared to those without (P<0.03). Posterior LSPV ostial ablation without damaging the anterior wall or LOM slowed residual LA-PV conduction (P<0.001). This diminished PV-LA coupling prevented the reinduction of LSPV focal activations in all dogs. However, persistent LOM focal activations in two dogs continued to activate the LSPV rapidly (CL 151.8 ± 4.8 ms) via direct LOM-LSPV connections. LOM-LSPV connection forms an accessory pathway that contributes to the electrical coupling between LSPV and LA during SR and AF. This pathway may contribute to rapid activations within the LSPV during AF.