Polymer-based engineering materials (plastics, polymer matrix composites, and similar) are becoming more widely used for the design and construction of consumer products and systems. While providing a host of design benefits, these materials also can have a large detrimental effect on the environment when not handled properly. One of the best ways to increase the sustainability of systems created using these materials is to extend their operating life as much as possible. Additive manufacturing (AM) technologies offer a powerful tool for this, as they allow easy repair of damaged or worn components in an automated or semi-automated way. This article explores the use of the fused filament fabrication (FFF) process as a tool for repairing high-value (i.e., difficult or expensive to replace) thermoplastic parts. The major design opportunities and restrictions are presented, as well as an evaluation of the types of repair jobs for which this process could be suitable and effective. Advice and ideas for future implementations and improvements are provided as well. A detailed case study is presented, where cracked ABS bars were repaired using FFF-deposited patches while varying the print parameters using a factorial designed experiment. The repaired bars were tested against the baseline and in most cases were found to be as good as or better than the original bars under a bending load. This case study demonstrates the concepts and explores how this repair approach could be realistically employed in practice.