While aptitude has been seen to contribute to second language (L2) development, most studies have examined its contribution as a fixed trait, rather than viewing it as a dynamic, changing variable. There appears to have been little or no investigation of the role that aptitude plays in the development of L2 pragmatic ability. Therefore, this study sought to investigate the contribution of L2 learning aptitude as measured by Grigorenko et al.'s (2000) CANAL-FT ("The Cognitive Ability for Novelty in Acquisition of Language as Applied to Foreign Language Test") to L2 speech-act knowledge among 121 Iranian upper-intermediate to advanced level EFL learners. In addition, a multiple-choice discourse completion test (MDCT) comprising three common English speech acts (Derakhshan, 2014) was administered to the participants. A multiple regression analysis revealed that all five components of CANAL-FT were significant predictors of L2 speech-act knowledge. The aptitude for learning sentential inferences was the strongest predictor, followed by the ability to acquire the meanings of the whole passage. The other three components of aptitude, namely, comprehending the meanings of contextualized neologisms, acquiring the language rules, and learning the meanings of paired associates, were found to be moderate predictors of L2 speech act knowledge. These results appeared to underscore the benefits of obtaining aptitude data on learners as a means for better understanding the dynamics of L2 development in the area of pragmatics.