Abstract: A flex duct fitting system includes: a flex duct with an inner lining, a flex duct fitting device, including a connector portion, including a connector portion body, a flex fitting assembly with a peripheral protrusion and a peripheral indentation; a circular fastener, such that the inner lining is pulled over the flex fitting assembly, such that the at least one circular fastener is positioned in the peripheral indentation and tightened around the connector portion body, with the inner lining tightened in place between the at least one circular fastener and the peripheral indentation.