Critically ill patients admitted to the intensive care unit (ICU) frequently develop new-onset atrial fibrillation (NOAF) due to numerous risk factors. While NOAF has been associated with increased mortality, it remains unclear whether it serves merely as a marker of illness severity or directly contributes to adverse outcome. This study aimed to determine the incidence and risk factors for NOAF in a homogenized population of mechanically ventilated patients at ICU admission, excluding well-established predisposing factors. Additionally, we examined the impact of NOAF on mortality in this context. We prospectively studied consecutive patients over a 3-year period to identify triggers for NOAF. Factors associated with 30-day mortality during the ICU stay were recorded. Demographic data, medical history, laboratory findings, and the severity of illness at admission were compared between patients who developed NOAF and those remaining in sinus rhythm. In NOAF patients, the course of atrial fibrillation (resolution, persistence, or recurrence) was evaluated during the 30-day ICU stay. Of the 1330 patients screened, 685 were eligible for analysis, with 110 (16.1%) developing NOAF. Septic episodes occurred more frequently in the NOAF group compared to the no-NOAF group (92.7% vs. 58.1%, < 0.001). Notably, 80% of NOAF patients developed a septic episode concurrently with the atrial fibrillation, often stemming from secondary infections, and 85.3% presented with septic shock. When focusing on patients with at least one septic episode during the 30-day ICU stay, 23.4% of them developed NOAF. Additionally, patients with NOAF were older and had a higher prevalence of hypertension; disease severity at admission was not a triggering factor. Mainly sepsis, but also advanced age, and a history of hypertension remained independent factors associated with its occurrence. Sepsis, primarily, along with advanced age and a history of hypertension, was identified as independent factors associated with the occurrence of NOAF. Mortality was higher in the NOAF group compared to the control group (39 patients (35.5%) vs. 138 patients (24%), = 0.01). NOAF occurrence, sepsis, disease severity at admission, and age were associated with increased ICU mortality; however, NOAF was not found to be an independent predictor of ICU mortality in multivariate analysis. Instead, sepsis, age, and disease severity at admission remained independent predictors of 30-day mortality. Sinus rhythm was restored in 60.9% of NOAF patients within 48 h, with the improvement or stabilization of sepsis being crucial for rhythm restoration. NOAF is a common complication in intubated ICU patients and is independently associated with sepsis, advanced age, and hypertension. While NOAF is linked to increased ICU mortality, it is more likely a marker of disease severity than a direct cause of death. Sepsis improvement appears critical for restoring and maintaining sinus rhythm.