Despite the importance of water vapor especially in the tropical region, the diurnal variations of water vapor have not been investigated in the past due to the lack of observations. Measurements from Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie (SAPHIR) onboard the low inclination Megha-Tropiques satellite with frequent daily revisits provide a valuable dataset for investigating the diurnal and spatial variation of tropospheric relative humidity in the tropical region. In this study, we first transformed SAPHIR observations into layer-averaged relative humidity, then partitioned the data based on local observation time into 24 bins with a grid resolution of one degree. Afterwards, we fit the Fourier series to the binned data. Finally, the mean, amplitude, and diurnal peak time of relative humidity in tropical region were calculated for each grid point using either the measurements or the Fourier series. The results were separately investigated for different SAPHIR channels as well as for relative humidity with respect to both liquid and ice phases. The results showed that the wet regions are normally associated with convective regions, and the dry regions with the high pressures. The new analysis reveals new findings for the diurnal amplitude and peak time of tropospheric relative humidity. The results showed a large inhomogeneity in diurnal variation of tropospheric relative humidity in tropical region. Although, an early morning peak-time is observed over some parts of tropics, there are significant regions where the diurnal peak time occurs at other times of the day. The results also showed that the diurnal amplitude is less than 10 % in middle and upper troposphere, but it is up to 30 % in lower troposphere over land.