Abstract: A study was conducted to assess seasonal variation in physicochemical and microbiological quality of groundwater in Ruiru, Kiambu County in Kenya. A total of 109 well water samples, 51 boreholes and 58 shallow wells during 2 different seasons, viz. dry and wet season were collected during the year 2015 and analyzed for temperature, pH, colour, electrical conductivity, turbidity, total hardness, calcium, magnesium, Sulphates nitrate, fluoride, iron, manganese, coliform count and Escherichia coli Significant difference between seasons was observed in all parameters studied except, for sodium and magnesium. The results were compared with WHO guidelines, 2009 and Kenya Bureau of Standards 2006 water guidelines to ascertain its suitability for domestic use.