Background and objective: Paediatric malnutrition has a synergistic relationship with diarrhoea. In children under two years of age, diarrhoea occurs in more than half of malnutrition cases and is associated with increased duration of illness, increased length of hospital stays, increased morbidity, and mortality. A well-established relationship exists between diarrhoeal and various standalone forms of malnutrition, but their association with coexisting forms of malnutrition (CFM) has not yet been investigated. Thus, this study assessed the association of CFM with diarrhoea among Pakistani children using datasets retrieved from Demographic Health & Survey and UNICEF. Study design: A pooled analysis of datasets of Pakistan Demographic & Health Surveys (PDHS) and Multiple Indicator Cluster Surveys (MICS) from the year 2010 to 2018 was conducted. Methods: Data of 70,723 children aged below two years were analysed after excluding those with incomplete anthropometry and outliers. Findings: Of the total study population, this study reported the presence of either diarrhoea or malnutrition or both in over half of Pakistani children aged between 0 and 23.9 months. Both standalone forms of undernutrition and coexisting forms of undernutrition were significantly associated with increased odds of diarrhoea by 1.07 (1.02 to 1.12) and 1.21 (1.16 to 1.27) times. The practice of breastfeeding, secondary/higher level of maternal education, and improvement in socioeconomic status reduced the risk of diarrhoea, while the children aged between 6 and 23.9 months residing in urban areas showed a high risk of diarrhoea. Conclusions: The presence of any type of undernutrition, i.e., standalone and/or coexisting forms were associated with increased risk of diarrhoeal diseases in children. An improvement in socioeconomic status, adherence to the continuation of breastfeeding, and maternal education are keys to reducing the burden of diarrhoea among children.