Purpose: The aim of this study was to assess the effect of Ramadan fasting on choroidal thickness (CT) and its associated diurnal variations using spectral domain optic coherence tomography. Materials and Methods: Ocular measurements were performed on a single eye of 87 healthy individuals. Eyes were evaluated twice a day around 8.00 a.m. and 4.00 p.m. during Ramadan. Evaluations were repeated at the same time of the day, 1 month following Ramadan on the same subjects. Measurements of choroidal and retinal thickness were performed at central, temporal and nasal segments. Results: The comparison of measurements revealed that temporal CT at 8.00 a.m. and foveal, temporal and nasal CTs at 4.00 p.m. were significantly reduced during fasting (p values were 0.005, <0.001, 0.001 and 0.046, respectively). Mean changes of CTs during fasting were 14.76 ± 16.01, 9.90 ± 19.57 and 8.10 ± 16.81 µm at the fovea, temporal and nasal segments, respectively. However, CTs at non-fasting conditions were measured as 6.13 ± 12.93, 3.78 ± 15.23 and 5.05 ± 13.60 µm, respectively. In comparison, diurnal variations of foveal and temporal CTs during fasting were significantly higher than the controls (p values were <0.001 and 0.004, respectively). Additionally, retinal thicknesses during fasting were significantly reduced compared to the controls in all segments measured at 4.00 p.m. (p for all values <0.05) Conclusions: The results of the present study revealed that fasting and dehydration caused a reduction of the choroidal and retinal thicknesses. Likewise, they are also responsible for the increased diurnal variation of CTs. Therefore, early hours of the day may be preferred to evaluate retinal and choroidal thicknesses during Ramadan to avoid the effects of dehydration on the measurement results.