Background: Laparoscopic sleeve gastrectomy (LSG) is increasingly being applied to treat obesity. LSG includes excision of the normal gastric pacemaker, which could induce electrical dysrhythmias impacting on post-operative symptoms and recovery, but these implications have not been adequately investigated. This study aimed to define the effects of LSG on gastric slow-wave pacemaking using laparoscopic high-resolution (HR) electrical mapping. Methods: Laparoscopic HR mapping was performed before and after LSG using flexible printed circuit arrays (64–96 electrodes; 8–12 cm2; n = 8 patients) deployed through a 12 mm trocar and positioned on the gastric serosa. An additional patient with chronic reflux, nausea, and dysmotility 6 months after LSG also underwent gastric mapping while undergoing conversion to gastric bypass. Slow-wave activity was quantified by propagation pattern, frequency, velocity, and amplitude. Results: Baseline activity showed exclusively normal propagation. Acutely after LSG, all patients developed either a distal unifocal ectopic pacemaker with retrograde propagation (50%) or bioelectrical quiescence (50%). Propagation velocity was abnormally rapid after LSG (12.5 ± 0.8 vs baseline 3.8 ± 0.8 mm s−1; p = 0.01), whereas frequency and amplitude were unchanged (2.7 ± 0.3 vs 2.8 ± 0.3 cpm, p = 0.7; 1.7 ± 0.2 vs 1.6 ± 0.6 mV, p = 0.7). In the patient with chronic dysmotility after LSG, mapping also revealed a stable antral ectopic pacemaker with retrograde rapid propagation (12.6 ± 4.8 mm s−1). Conclusion: Resection of the gastric pacemaker during LSG acutely resulted in aberrant distal ectopic pacemaking or bioelectrical quiescence. Ectopic pacemaking can persist long after LSG, inducing chronic dysmotility. The clinical and therapeutic significance of these findings now require further investigation.