Aim: The study aimed to investigate the association between time-domain heart rate variability (HRV) parameters, glycated hemoglobin (HbA1c) levels, and the duration of Type 2 Diabetes Mellitus (T2DM) to assess the progression of autonomic dysfunction. Material and Methods: This prospective observational study included 120 adult patients with T2DM aged 30 to 70 years. Time-domain HRV parameters, including SDNN, RMSSD, NN50, and pNN50, were measured using a 5-minute electrocardiogram (ECG). HbA1c levels and T2DM duration were recorded to evaluate glycemic control and disease progression. Follow-up assessments were conducted over 12 months. Pearson correlation and linear regression analyses were used to examine associations between HRV parameters, HbA1c levels, and diabetes duration. Results: The mean SDNN was 25.3 ± 6.7 ms, and RMSSD was 18.6 ± 4.9 ms, both reflecting reduced HRV. Significant inverse correlations were observed between HRV parameters and HbA1c levels (SDNN: r = -0.48, RMSSD: r = -0.52, p < 0.001) as well as the duration of T2DM (SDNN: r = -0.62, RMSSD: r = -0.58, p < 0.001). Over the 12-month period, HRV parameters showed a significant decline, with SDNN decreasing by 9.88% and RMSSD by 12.90%, indicating progressive autonomic dysfunction. Conclusion: This study demonstrates a strong association between reduced HRV parameters, poor glycemic control, and longer T2DM duration, highlighting the progressive nature of autonomic dysfunction in diabetes. Regular HRV monitoring and optimal glycemic management are essential to mitigate cardiovascular risks and improve patient outcomes.