Offshore facilities, systems, and methods for treating wastewater have been developed. An offshore wastewater treatment facility include wastewater treatment components (for preliminary treatment, primary treatment, secondary treatment, and/or advanced treatment); a means for moving wastewater from a land-based site to the facility; a means for removing treated wastewater from the facility; and a means for removing sludge from the facility. A method of treating wastewater includes: transporting wastewater from a site on land to a wastewater treatment facility located offshore in a body of water, and subjecting the wastewater to preliminary treatment, primary treatment, secondary treatment, and/or advanced treatment.