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Background The complex relationship between heart failure and atrial tachyarrhythmias (AT/atrial fibrillation [AF]) is not well understood. We examined the temporal association between changes in intrathoracic impedance, suggesting thoracic fluid accumulation, and AT/AF occurrence in cardiac resynchronization therapy-defibrillator and implantable cardioverter defibrillator (ICD) patients. Methods A retrospective analysis was conducted on stored implantable device data to identify patients with automatic monitoring of daily AT/AF burden and intrathoracic impedance. Daily population trends in AT/AF burden before and after a fluid index threshold crossing (FIC) were determined. Results A total of 73,018 patients (68 ± 12 years, 51% ICD, 75% male) were evaluated over 18.6 ± 11.5 months. Kaplan-Meier analysis indicated a significantly higher probability of FIC events in the first month following the onset of persistent AT/AF when compared to a matched group without persistent AT/AF (hazard ratio [HR] 1.65, 95% confidence interval [CI] [1.58, 1.72], P < 0.001). Conversely, patients were significantly more likely to experience an episode of persistent AF in the first month after the FIC event (HR 1.32, 95% CI [1.08, 1.63], P = 0.008). The probability of a fluid index crossing within 30 days of the onset of persistent AT/AF was significantly lower in a subgroup of patients with adequate rate control (35.8% [34.3–37.4%] vs 42.0% [39.6–44.6%]; HR 1.24 [1.13–1.36]). Conclusion Thoracic fluid accumulation, as indicated by decreasing intrathoracic impedance, was more likely to occur immediately after the onset of persistent AT/AF, especially in the presence of inadequate rate control. Likewise, the onset of persistent AT/AF was more likely following a decrease in intrathoracic impedance.