The present study is aimed at investigating the effect of portland cement fineness on the results of ASTM C1260 tests—Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method). The effects of clinker alkali content, aggregate reactivity, and sodium hydroxide (NaOH) solution concentration on the mortar expansion test results were also studied. In this study, high and low alkali portland cement clinkers were selected and ground with gypsum in a laboratory ball mill to three fineness levels: 300, 400, and 500 m2/kg (Blaine). Moderately- and highly-reactive aggregates were tested with these cements according to ASTM C1260. In addition to the standard 1 N NaOH solution, a 0.5 N NaOH soak solution was also used. The results show that mortar-bar expansion was promoted with increased cement fineness regardless of clinker alkali, aggregate reactivity, or soak solution normality. Clinker alkali had no or little effect on moderately reactive aggregate; whereas it had considerable effect on highly reactive aggregate. Highly reactive aggregate tended to be more sensitive to cement fineness and alkalinity. Although cement fineness and clinker alkali content did not affect the classification in terms of potential reactivity of the aggregates used in this study, they might change the classification for the aggregates near the boundary between reactive and nonreactive aggregates. Specification of fineness and alkali content of cement, or both, might be beneficial to further improve the reliability and reproducibility of ASTM C1260.