Klebsiella pneumoniae is a human, animal and environmental commensal and a leading cause of nosocomial infections, which are often caused by multi-resistant strains that are challenging to treat. We conducted a One Health evaluation of putative sources of K. pneumoniae that are carried by, and infect hospital patients. This combined data from a six-month study on two haematology wards at Addenbrooke’s Hospital, Cambridge, in 2015 to isolate K. pneumoniae from stool, blood and the environment, and a cross-sectional survey of K. pneumoniae from 29 livestock farms, 97 meat products, the hospital sewer and 20 municipal wastewater treatment plants in the East of England between 2014 and 2015. K. pneumoniae was isolated from stools of 17/149 (11%) patients and 18/922 swabs of their environment, together with one patient bloodstream infection during the study and 4 others over a 24-month period. Each patient carried one or more lineages that was unique to them, but two broad environmental contamination events and patient-environmental transmission were identified. K. pneumoniae was isolated from cattle and poultry, hospital sewage and 12/20 wastewater plants. There was low genetic relatedness between isolates from patients/their hospital environment versus isolates from elsewhere. Identical genes encoding cephalosporin resistance were carried by isolates from different reservoirs, but were carried on different plasmids by isolates from patients/their environment versus elsewhere. We identified no patient-to-patient transmission and no evidence for livestock as a source of K. pneumoniae infecting humans, but our findings reaffirm the importance of the hospital environment as a source of K. pneumoniae associated with serious human infection.