Microplastic pollution in marine waters around the globe is increasing exponentially. This is the first comprehensive review which focuses on microplastics as a source and vector for metals, antibiotics, toxic chemicals, pathogenic bacteria (Vibrio cholerae), and Harmful Algal Bloom (HAB)-forming dinoflagellates across the continents through ballast water. Microplastics in ballast waters serve as ‘hotspots’ for the development and spread of multiple drug-resistant human pathogens through co-selection mechanisms. Microplastic inoculation at distant countries through ballast water may pose a serious threat to human health due to higher incidences of bacterial disease outbreaks and HABs. The 2017 ballast water management convention lacks a provision for on-board treatment of microplastic-contaminated ballast water. We conclude that there is a pressing need to include microplastics in the ballast water management convention as a hazardous material. Efficient on-board ballast water treatment strategies and effective limits for microplastics in ballast waters need to be developed. Display omitted •Metals, chemicals and antibiotics adsorb on microplastics in contaminated waters.•V. cholerae and HAB-producing species form biofilms on microplastics.•Ballast water transports microplastics and ‘plastisphere’ communities across continents.•Disease outbreaks and Harmful Algal Blooms can occur in the recipient country.•Ballast water convention should include microplastics as a hazardous material.