This study aimed to assess the knowledge, attitudes and practices on nutrition among mothers with under five years undernourished children in Kicukiro District of Rwanda. Malnutrition constitutes a big threat to the public health especially among children less than five years and it is a burden in developing countries. The study used a cross sectional design with quantitative and qualitative approaches. A sample of 255 mothers living in Kicukiro District who have undernourished children below 5years were chosen using a convenience sampling process, whereby every mother that had an undernourished child was given equal chance to be enrolled in the study. Data analysis was done using SPSS version 22. Qualitative data were treated using Atlas. Univariate, bivariate and multivariate analysis were applied.