Wastewater treatment sludge (WTS) is a by-product of wastewater treatment. Moisture holding capacity, high organic, complex inorganics, and zeolite-like chemical contents make WTS a valuable pavement additive. This study evaluated the impacts of 1.5% WTS on fatigue cracking and low-temperature cracking resistance of asphalt mixes. Reclaimed asphalt pavement (RAP) from North Dakota Highway-17 and I-29 projects were used. Three mixes—0%, 40%, and 60% RAP content—with and without WTS, were considered. The rheology of extracted RAP was tested for high-temperature continuous temperature, intermediate-temperature fatigue cracking resistance using linear amplitude sweep (LAS), and low-temperature cracking resistance using 4-mm parallel plate geometry. Rheology tests were conducted using a dynamic shear rheometer (DSR). Results showed that RAP from Highway-17 was relatively stiffer than the I-29 RAP. RAP mixes modified with 1.5% WTS were prepared at 40°C (104°F) lower mixing temperature than the control. Semicircular bend (SCB) and disk-shaped compact tension (DCT) tests were used to evaluate the fatigue cracking and low-temperature cracking resistance of the mixes, respectively. Results indicated that WTS improved the fatigue and low-temperature cracking resistance of 0%, 40%, and 60% RAP mixes from both RAP sources. The effect of WTS on fatigue cracking resistance increased with RAP content, whereas the effect of WTS on low-temperature cracking decreased with RAP content. Low-temperature cracking resistance of the 40% RAP mix was 24% higher than that of unmodified 0% RAP mix for Highway-17 mixes, and was almost similar for I-29 mixes. This shows WTS can be used as an additive to improve fatigue and low-temperature cracking resistance of mixes with RAP.