The Saint John (Wolastoq) River (SJR) is a major international river on the eastern seaboard of North America; one of the largest rivers in terms of river length and basin area on the eastern seaboard of North America south of the St. Lawrence River system. The geography, history, and characteristics of the Saint John River basin (SJRB) are briefly discussed. Subject to seasonal ice cover, ice conditions vary in the SJRB due to natural differences in climate and terrain, and due to anthropogenic changes, such as the construction of dams. An overview of typical winter climate and ice-season characteristics and trends in relevant hydroclimatic variables are discussed. The SJR itself is then described. Spring freshet and midwinter ice breakup has created ice runs, ice jams, and damaging ice-related flood events along the SJR and its tributaries that have resulted in loss of life, property damage, and loss of, or damage to, infrastructure. A synopsis of past ice jamming and associated flooding along the main stem of the SJR is presented. Ice formation, growth, and breakup along the SJR affects its hydraulic and ecological regimes, and thereby erosion, ecology, water quality, recreational uses, and flooding. Projections of future ice regimes under a changing climate are provided, with anticipated changes in the ice regime of the SJR explained in the wider context of changes to the cryosphere. For the SJR and other northern rivers, the ice season should not be ignored in river engineering and basin management.