The coefficient of thermal expansion (CTE) of concrete has substantial effects on the behavior and performance of portland cement concrete (PCC) pavement. The CTE is one of the input variables with significant effects on PCC pavement performance in the newly developed Mechanistic-Empirical Pavement Design Guide. Currently, the most advanced and accepted evaluation method for the CTE is the provisional AASHTO TP60. In this test method, concrete specimens are saturated before and during the testing. It has been recognized that the CTE of cement paste is influenced by the relative humidity (RH) within the specimen. Results from previous research studies were nearly consistent: the maximum CTE value occurs at about 70% RH and its value is almost twice the value at 100% RH. Laboratory evaluations were conducted to quantify the effects of RH on CTEs in cement paste and concrete. In the testing program, target RH levels within the specimens were 45%, 60%, 70%, 80%, and 100%. RH sensors were installed within the specimens during their preparation. The specimens were fabricated and cured in 23°C (73°F) and 50% RH conditions for 6 weeks. Subsequently, the specimens were placed in the environmental chamber until the internal RH values reached the target RH levels. Then CTE testing was conducted by changing temperatures while evaluating displacements with externally mounted vibrating wire gauges. The results showed some effects of RH on the CTE of cement paste and concrete, with maximum values at about 70% to 80% RH. The effect was larger for cement paste than for concrete. Considering the small effects of RH on the concrete CTE, AASHTO TP60 appears to provide adequate CTE values for PCC pavement analysis for environmental stresses.