<p>Analysis of Tanzanian precipitation data from 1980 to 2020 reveals dominant annual and semi-annual rainfall cycles. This study applies harmonic analysis to identify rainfall periodicity in Tanzania. The annual cycle (first harmonic) explains over 10% of rainfall variability nationally, exceeding 40% in many areas, notably the southern, southwestern highlands, western, and central regions, likely due to the Congo air mass. The northern coast, northeastern highlands, and Lake Victoria basin show weaker annual influence due to local weather patterns. Unimodal rainfall regions display a more pronounced annual cycle than bimodal regions. Based on the first harmonic, maximum rainfall occurs in January, February, and March, especially in the southern Lake Victoria basin, western, central, southwestern highlands, and southern regions. The semi-annual cycle (second harmonic) is less dominant, particularly in the southern and western regions, often complementing the annual cycle. It is more pronounced in areas influenced by orography, lake breezes, and maritime air. The second harmonic amplitude is higher in bimodal regions, particularly around Mt. Kilimanjaro, Pemba, and northern Unguja. In these regions, the peak of semi-annual rainfall often occurs in March and April. The proportion between the first and second harmonic amplitudes indicates how much annual cycles dominate compared to semi-annual rainfall variations. In the southern, central, western, and southwestern highlands, the annual signal dominates and closely aligns with the first harmonic pattern. By contrast, the northern coast, eastern sector, and western Lake Victoria basin show a weaker degree of seasonality. The dominant first harmonic in the western, central, and southern coast regions signifies a strong single annual rainfall variation. The findings of this work in the characterisation of rainfall variability in Tanzania are essential for agricultural planning, water resource management, and disaster preparedness.</p>