An apparatus for thermally joining thermoplastic fiber composite components includes a pressurization arrangement for jointly covering, at least in a region of a joining zone, thermoplastic fiber composite components to be joined and applying pressure to the thermoplastic fiber composite components to press the thermoplastic fiber composite components against one another, at least in the joining zone, the pressurization arrangement being flexible, at least in some section or sections. A welding device is configured for welding the fiber composite components in the joining zone during pressurization. The pressurization arrangement and welding device are configured to weld the thermoplastic fiber composite components in a pressurized state in the joining zone. The pressurization arrangement is configured to maintain pressurization independently of the welding device until the joining zone solidifies. A cover is also disclosed for a pressurization device for thermally joining thermoplastic fiber composite components.