Purpose: We explored alphabet supplementation as an augmentative and alternative communication strategy for adults with minimal literacy. Study 1's goal was to teach onset-letter selection with spoken words and assess generalization to untaught words, demonstrating the alphabetic principle. Study 2 incorporated alphabet supplementation within a naming task and then assessed effects on speech intelligibility. Method: Three men with intellectual disabilities (ID) and low speech intelligibility participated. Study 1 used a multiple-probe design, across three 20-word sets, to show that our computer-based training improved onset-letter selection. We also probed generalization to untrained words. Study 2 taught onset-letter selection for 30 new words chosen for functionality. Five listeners transcribed speech samples of the 30 words in 2 conditions: speech only and speech with alphabet supplementation. Results: Across studies 1 and 2, participants demonstrated onset-letter selection for at least 90 words. Study 1 showed evidence of the alphabetic principle for some but not all word sets. In study 2, participants readily used alphabet supplementation, enabling listeners to understand twice as many words. Conclusions: This is the first demonstration of alphabet supplementation in individuals with ID and minimal literacy. The large number of words learned holds promise both for improving communication and providing a foundation for improved literacy.