Rainfall is the most important source of water for crop production in Ethiopia. However, its temporal and spatial variability is leading to serious food shortages and insecurity in the country. This study was aimed at investigating the characteristics of selected agroclimatic variables over the great Rift Valley regions of Ethiopia. Long term (1981–2010) climate data were analyzed for 17 stations selected based on agroecology representation. Selected descriptors for climate variability and the Mann–Kendall trend test were employed. Onset, cessation, length of growing period (LGP), water requirement satisfaction index and dry spell occurrence during the growing period were determined. The results showed low to very high rainfall variability (14–35%), LGP (20–256 days) and dry spell probability (50–100%) during the main season. Significant (P ≤ 0.05) annual and seasonal rainfall trends were observed in some stations. The probability of occurrence of a dry spell during the seasons was found to be a challenge for most of the stations in the mid and low altitude areas of the basins. Consequently, seasonal water deficit was observed in these areas which hampered crop production. Area specific recommendations are thus required based on specific challenges in the study region.